Carb or timing help...

I have a 1977 Chevy c10 half ton, LWB, trailering special. I got it from my uncle about 3 months ago after sitting for 5 years. To get it running, I changed the oil, spark plugs, fuel pump, battery, tranny pan gasket and new tranny oil. I took it to my house and would run it daily so the old gas would burn out of the system. On the third day the gas finally ran out and I added fresh new gas. After doing so, The truck didn't start anymore. It sat for two weeks until I changed the fuel pump again because the rod inside had fallen and wasn't pushing the manual pump. It ran again for one day and died again. Just a week ago, I put on a rebuilt carb and adjusted the timing a little(not with a light). I changed the plug wires and flushed my radiator.

Today, It might turn on but once it's in gear, It will sputter and die. If I let it idle, it'll idle for about 30 minutes and die for the rest of the day. This truck is all original and doesn't leak besides coolant. I don't want to put little money into this truck if It is gonna need a rebuild. I wouldn't mind buying little stuff if It were a strong engine though. I just want to know why it dies when it's in gear. I've heard it backfire from the exhaust and backfire from the carb. Any ideas? Would timing have anything to do with the sputtering only when in gear?