Truck runs fine but dies when you put it into gear, when I power brake it it will stay running but right as I let off the gas in gear it dies , rebuilt the carb , timed and adjusted , smoke and vac tested it no leaks I just don't know it is a 1978 chevy 2500 5.7 auto any I deas?

On this old lovely, check for worn or chaffed wiring within the distributor. Check also for a weak coil and or low fuel volume and/or fuel pressure.
There could be more than one issue with this type of vehicle. Kind of old school things could creep into diagnosis.
Have you done much to this rig already?

On this old lovely, check for worn or chaffed wiring within the distributor. Check also for a weak coil and or low fuel volume and/or fuel pressure.
There could be more than one issue with this type of vehicle. Kinda old school things could creep into diagnosis.
Have you done much to this rig already?
I replaced the distibutor, rebuit the carb, tested fuel pressure check for vac leaks, could it possibly be the tourque converter?

i replaced the distibutor, rebuit the carb, tested fuel pressure check for vac leaks, could it possibly be the tourque converter?
That's easy enough to check.
Unbolt the torque converter from the flywheel and slide it gently back to clear the back of the flywheel and start the engine. Do the symptoms remain?
Was there an issue with the transmission before the engine work? Any work done to the trans?
Seems odd...