ahh the good old 4.3
ok first question !
what engine series is this ? is it pre vortec ?
check the engine pan, is it steel or aluminum ?
im asuming its propane fuel ?
if so what hours has it done since the valves were done ?
one of the most common problems with early 4.3 gm engines were the exhaust valve seats burning out, the heat the propane developes in the chamber far exceeds a gas / petrol fueled engine.
under load it gives the impression of backfiring but in almost all cases when this engine runs on propane and developes such a fault it was the exhaust valve seats
if you get the heads done be sure the exhaust seats are stelite material,
a quick compression check may or may not reveal this.
just one more thing be sure the ignition wire ends are coated with silicon grease inside the plug boots its a must for hei ignition systems