If you have changed carbs and are still stalling I think it is time to forget the carb and look at something else!!

Have you checked that you are getting plenty of fuel to the carb?

My money is still on something in the ignition system, either the coil or condenser if it has one. Sometimes it is nothing more than a loose connection in the primary ignition circuit. Happened to me on my dump truck, the wire into the ballast resistor was just loose enough to kill the engine once in a while.

Last but not least, if you don't have a repair manual for this truck you should get one. There were some important changes to GM engine tuning specs in the mid '70s not the least of which was spark plug gaps, timing advance. GM was trying to comply with emission standards and get the engines to run well, didn't do very well at either.

'31 Chevy 1 1/2 ton,
'42 GMC 2 1/2 ton 6x4,
'52 GMC 2 1/2 ton 6x6,
'53 GMC 5 ton