whats the cause for backfire thru exhaust while shifting??

[jasonroman]

picked up an 83 c10.
st8 six. 4 spd.
notice that when shifting gears...not all the time, but a good majority of the time, i here a backfire type noise. only happens when switching gears.
had a new cat put on. my old one was hollow and muffler was beat up.
so i had a new cat put on and a muffler i had on my 85 welded up.
money is tight now. lost home and 2 vehicles in storm.
anyway so i had new cat put on and old/newer muffler put up. no piping from muffler
guy gives me back truck and tells me that its still backfiring. could be distributer or carb?? he says

so i tuned the truck up. and still have it.
timing is a little advanced right now. but not much. is that whats causing my pops?
or do i have to figure out how to mess with the carb?
or could it be something else??
appreciate any imput.