I have swaped 350 into a jeep cj7 I have wired it up and had it running, but the other day I went to start it and the engine started but when the key was released it died. I have checked the coil, OK, I replaced the ingnition switch, no luck. I would appreciate any ihelp on this subject. Thanks.  

i take it that you a big cap hei......

take the top coil cover off so you can get to the wires..... use  a test light on the one that is hooked to battery supply wire.... and watch that..

it is possable that you have lost part of the circut...   you did not say what year the cj7 was.. if it had points with the last motor.    the power to the coil positive  is supplied my 2 differnt sources... the starter solenoid or relay while the key is in the crank position.... and from the ignition switch in the run position...

most point type systems also have a resister wire or resister in the  run circut... and supply full power to the coil during cranking...  

You said yuo changed the ignition switch, what about the key lock & cylinder? Since the jeep dies when you let go of the key, there's a good chance that's where the problem is.  

might be ground wires  

need to find a hot source when key is let go i had the same problem with an msd 6al box i wired it up and when i turn the key to start it wouldnt start my dad told me that i needed to find a hot source when the key is released that stayed hot i know it sounds confusing but it works