its an auto. has a 350 chevy motor recently put in. it fires up and idles fine but stalls when it is put into gear. put new plugs wires, dis cap. fuel filter, oil filter etc.

 

I would suggest checking the fuel pressure. Not sure if it applies, but I was having a somewhat similar problem with my '95 sierra. The truck would start but usually start out. Feathering the gas would keep it going, but it would die once put in gear.

Weeks of troubleshooting was getting exhausting and expensive. Finally I stumbled on some advice on the fuel pumps. Since the truck was starting I ruled out a bad fuel pump, but apparently they can go bad but not completely stop working. This means that they can prime the engine and get it started but can't deliver enough fuel to maintain, expecially under load.

Rent a fuel pressure kit from your local auto parts store and test the lines and make sure they are putting out spec fuel pressure.

 

I'm going to assume it's carberated and NOT TBI. Check for vacuum leaks.
Old dried up vacuum lines, cracks or even connections gone loose will cause your truck to stall once the engine is put under a load. ie: put into gear