when i decided to go with electronic ignition i just took the 40 to the mechanic to have him do it, being that he was doing some wiring for me at the time. he told me he could get the GM HEI for half the price so he ordered it. it didn't work. he then ordered the DUI and i went over to see the difference between the two. not only does the DUI have a different gear at the end of the shaft, but the shaft of the DUI has been milled down in several places. the mech told me the diameter differences in the shaft were also necessary to make the DUI work. once the install attempt, he also noticed that the DUI shaft was slightly too long (apprehently this is not true in every case) so he had to place shims between it and the engine. RESULT: better performance, better mileage (barely), better throttle responce, lower idle. i don't think i would have gone ahead with this $400.00 upgrade if my distributor wasn't totally shot... but it was. if you are curious, pull the distributor and check the gear for wear and slop. if you have to replace your entire ignition because an "under the cap" overhaul still leaves you with terrible timing and inacurate dwell, then i would go with the DUI.