1984 GMC back fire when you let go of the gas

1984 GMC Sierra Classic 150,000 5.0 engine

The old carburator blew the catlitic converter and muffler I changed the carburator no I have a handme down holley straight form another engine and the truck still pops loudly when I release the gas going don the freeway, it sounds like cannon fire. Also when I rev up the engine in the drive way and release the gas it also pops loudly. I don't want to change the mufflers yet otherwise it could blow them again. Please Advise me on what it could be.
I just finished changing out the motor this past weekend it was smoking and it wasn;t going to pass the emmissions test. It was popping before the motor change and is still popping after the motor change. I installed a short block.

Thanks for you help with this. !