Need some help with a backfire problem

[Tim64]

Hey Guys,
Got a problem here a need some help. My truck is backfiring and loosing power when under a load. Like going up a hill. And it doesn’t have to be much of a hill. At low speeds on level ground it does fine. It starts good, and doesn’t die when it’s backfiring, but you sure feel like it’s going to.
When I first got the truck it had been sitting for a few years. The vacuum advance arm broke and left me stranded after about 100 miles. I put in a new distributor, had the gas tank cleaned out, and ran new fuel lines. Also had a friend rebuild the carb, which was a rebuilt one to begin with. He said he thought it was pretty much a POS but he did the best he could do. He’s a GM mechanic so I think he knows he stuff pretty good.
The thing is acting to me like it’s having too much gas dumped in it when under a load and the engine isn’t firing fast enough to keep up. The timing is set a factory specs. The vacuum advance is working, as I can see the arm move when I rev the engine, which also makes it backfire. Is it possible for the timing gears to skip, or be worn out like a chain might do? Maybe I should advance the timing some to make up for the worn condition.
Sure hope you guys can give some ideas I need to have this running by Monday morning.
Thanks, Tim