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.