In need of help please

[Flint70step]

My friend has a 66 c/20 with a 350/4 speed combo.This past weekend he went to start it in the morning and it would fire off but die as soon as he the key went back to the on position from the starting position.Before I noticed this we chased down everything else we could think of trying to find the culprit.Then I got out my test light and put it on the coil pos. post and told him turn the key to start the engine.The test light lit up while he cranked the motor over and it started for a sec then died as soon as the key went back to the on position.He got a new ignition switch about a month ago and we just got another today to see if that was the problem.Put in the new switch and its still doing the same thing.Any help would be greatly appreciated.Thanks

I did forget to mention that the truck has a MSD ready to run pro billet dizzy with an external coil.He had a stock style coil on but we put a blaster2 coil off my white truck on it to see if it was the coil.

[losthope]

the coil pos+. post should be hot while in the start and on position's , you said you tested and its hot in the start/cranking position, is it hot in the on/run postion?? I would check that first it sounds like your trying to start the engine but when the key goes back to the on/run position it dies because its not gettin power.....might not be the problem but by what your explaining thats what it sounds like...also the past two new ignition switch's that i bought from napa are both temperamental because they are cheap p.o.s's unlike the originals

[losthope]

I always use this simple way to check the coil's on my cars to see if they are working...If not i find myself swapping on 2 to 3 coils trying to see if any work and this way you can rule the coil out...just a thought:

Ignition Coil - How to Test - YouTube