maby its running to rich which could make the plugs faul and would make it not run how long can you keep it at idle without it dieing? check the jets and plugs is all i know of.
hope this helps and im happy your keeping the fenders and dumping it its gunna look bada**
i say do a small chop 2-3 inches max and just make it as low as possible without cutting up the frame but thats just me i just hate it when i see all these nice old trucks getting chopped up to the point you cant even tell what it was beafore they chopped it up. sorry but i think its to clean to...
are you using the 10bolt for this build if so have fun putting a new rear end in in a short time they dont hold up for almost any power or abuse i would just do a 14bolt.