It may help if you better describe your current system, ie. what's oem and what's not. And when you suggested you may never have had 12v at coil+, what does that mean? Have you ever had your 79 running? Details would help here.
However, if...your 79 is still oem wrt ignition and bypass wiring, then yes, you should get 12v from the starter at the coil+ when cranking. The 79 schematic will confirm that. However, it should not continue to crank without the switch on (or with the switch unplugged).
If you also...don't get battery voltage at the coil+ with the ignition switch on, then that's another problem. Kinda strange you have both problems at the same time, maybe related?
You may want to think back to when everything worked as it should, to isolate what was done to the system to make things not work as it should. If something was changed, and then it stopped working properly, that's probably the source of your problems.
It seems like wires or connectors may have been swapped somewhere. Have a close look at your starter connections to make sure they are correct. One of those wires (heavier BW - black with white stripe) from the ignition switch, should have 12v on it only when the switch is in the "start" position. The other wire (also BW, but smaller gauge) goes to the to the coil+ ( bypass circuit).
On the coil+ you should also have a BY (black with yellow) wire from your ignition switch via your in-harness resistor wire. That should have 12v on it when the switch is "on".
If any of those voltages aren't there, then you'll have to trace those circuits back to where you do have the proper voltages.
You might also have a close look at your fuse block wiring, following the schematic and color coding. Maybe two of those terminals got swapped?
Look for simple things first, and as was mentioned previously, try to stick to the problem at hand, rather than chase red herrings and risk complicating things further.
Good luck!
