Well the coil is part of the ignition system. It's good practice when replacing a part of a system like a new dizzy, to replace everything in that system at the same time. All I know is I was having problems with mine similar to yours and I replaced the carb, dizzy, coil, plugs, and wires and my problems went away.
You have good comp.#'s, no cracks in the intake, carb isn't leaking fuel. I'm just saying if it were me, I'd sell off the suspect parts on ebay or craigslist and put the $ towards all new parts and start fresh.
Got your email, yes , it's very doable.
I've had my DUI setup for over seven year and it has worked without any issues regardless of the speed I run. My vehicle climbs grade at a higher mph than it did with my factory setup which also worked without any issues. If your 40 idles then I question that your problems are due to the DUI. I've never seen a distributor that would function at idle and then die at higher rpm's and then function again at idle. Obviously anything is possible.
The following is from Performance Distributors and is worth knowing. Even though the distributor appears to work at idle the following will let you test two of the three electronic components and you can buy and replace the module if you cannot find an auto parts store capable of testing a GM HEI module.
Distributor Will Not Fire
If you are having a problem with the distributor not producing a spark, then follow these steps to help determine the problem. First and foremost, check to make sure your hot wire is at least 12 gage from the battery to the fuse panel to the ignition key and finally to the distributor and make sure there are no resistors inline (such as resistor wire used by Toyota inline to or from the fuse panel and I do not mean the fuseible link). Also, check to make sure all your grounds are good. There is a ground wire that attaches to one of the starter mounting bolts and then to the passenger side frame which has a threaded hole to mount that end of the ground strap. A bad ground will keep the distributor from firing. I would measure the resistance across the lugs of the ground strap and then from the starter to the frame itself to see if the attach points need to be cleaned. I would make sure the other ground strap from the battery to the frame also has low resistance. If the hot wire and grounds appear to be good, then the problem could be with one of the electronic components inside the distributor. The distributor has three electronic parts which make it fire. They are the magnetic pick up coil, the module and the coil. The magnetic pick up coil is located inside the distributor underneath the rotor and weight and spring assembly. It has a white and green wire coming out of it that plugs into the module with a yellow plastic insulator. The magnetic pick up coil can be tested using an ohm meter. With the green and white wires disconnected from the module, touch the meter leads to the terminals of these wires. The reading should be between 800-900 ohms. A normal reading is about 830-850. As long as it is within the 800-900 range, then the part is good. The coil (in the cap) can be tested with an ohm meter as well. The resistance values for the coil are as follows. To check the primary side of the coil, touch the meter leads to the red and yellow wires. The reading should fall between 0.3-1.0 ohms. Usually 0.6 - 0.9 ohms is normal. Note that the red wire is positive (+) and yellow is negative (-). To check the secondary side of the coil, it must be removed from the cap. Touch one lead to the center black wire and the other to the bottom of the coil where the rotor bushing makes contact. This reading should be between 6.00K-30.00K ohms. The normal reading is about 8.5K - 9.0K. Refer to illustration 1 for proper testing of the primary and secondary resistance of the coil. For the module, there is no resistance check that you can perform to see if it is good or bad. If you suspect the module is bad, you can replace it with another module to see if this will get the distributor to fire. A stock GM module can be used for this test. Or you could take the module to an auto parts store such as Auto Zone or NAPA and have them test it. It is recommended to have the test procedure performed 4-5 times. Reason is, each time the module is tested it develops more heat and heat is a major factor in the breakdown of the module.
Oil in the Top of the Distributor
Oil in the top of the distributor housing around the pickup coil and module is caused by too much crankcase pressure. The distributor does not suck oil up into itself. The shaft and gear only rotate; they do not force oil upward into the housing. The crankcase pressure forces the oil up into the distributor. If in fact you find there is an oil problem then you should correct the source of excessive crankcase pressure. If that option is not available then drill two oil relief holes in the distributor housing. First, remove the roll pin from the gear and remove the gear from the shaft. Be careful not to lose the shims and tang washer that are between the gear and housing. With the cap and rotor off, remove the shaft from the housing. On the housing itself, drill a 1/4" hole into one side about 2" up from the bottom of the housing. Do not drill all the way through, just to the center and back out. Make sure you drill above the two rings at the bottom of the housing. These are oil bosses and you do not want to drill into them. On the opposite side of where you drilled the first hole (180 degrees), drill another 1/4" hole about 1" underneath the flange that rests on the intake (where the hold down clamp is attached). Again, do not drill through to the other side, just to the center and then back out. These two holes will allow the oil to drain back down into the engine and will not hurt the strength of the housing.
Non Existing Distributor Installation
If you are starting fresh and installing a DUI Distributor in a new or rebuilt engine, then you will need to bring the number one cylinder up to top dead center (TDC) on the compression stroke. Make sure you are on the compression stroke or you will be 180 degrees out on the timing. If necessary, have someone spin the engine over while holding your thumb over the number one spark plug hole until you feel it "blow” and this will be your compression stroke. Turn the engine by hand to line the timing mark on the bell housing to the line in the flywheel below the BB timing mark. Once this is established, drop the distributor in the block (without the cap) and point the number one rotor just right of the #4 spark plug. Make sure the distributor engages the oil pump and that it bottoms out on the block. There is supposed to be a small gap between the end of the distributor shaft and the bottom of the slot in the oil pump. If the distributor does not bottom out on the block then you need to determine if it is a tight fit or that the HEI shaft has bottomed out on the oil pump slot. Do not run the engine until this has been corrected because damage to your hardware may result. Once the distributor is installed and all clearances have been checked (see Topic 1), re-install the cap. Now, add the spark plug wires and follow the firing order starting with the terminal that was designated as number one. For optimum performance, set the spark plug gaps at .050" - .055". Leave the vacuum advance disconnected and plug the vacuum line. Your engine should now be ready to start. Crank the engine and adjust the distributor until it fires. Once the engine is running adjust the initial timing to about 7 degrees (BB) before top dead center (BTDC) while it is idling at around 650 rpm’s. Once initial timing is set, lock the distributor down and recheck the timing. Plug in the vacuum line and go for a road test. Make sure there is no pinging under hard acceleration. If pinging is detected, retard the timing until it is eliminated.
In the photo of my set up you can see how much the HEI was rotated CCW to time the engine to 7 degrees or the BB.
(<--- pigger salute)