Chevrolet Repair: back firing, vacuum leak, valve train

Question
I have a 1985 Chevy 1 ton dually, I have taken the diesel engine out a couple of years ago. I now have a 454 no emissions or anything. The truck runs great and have have used it to pull or carry anything. But lately whenever I have a load on it 700 pounds or more it back fires into the air filter at least that's my guess because of the sound and the underside of my filter is black. It only does it going up a long hill couple of miles. And when it starts it well only do it once but the longer the hill the worse it gets. If you let the truck sit it well be OK and run right. Without a load there is no problem at all it well even spin the tires but under a load it backfires going up hill great on the flat. Am lost a don't know where to start.

Thanks a lot.

Answer
Things that cause backfires; lean conditions, vacuum leak(s), low fuel pressure, cracked dist. cap, bad wires(crossfire(s), cracked spark plug(s), valve train problems, such as; weak springs, badly worn rockers/pushrods, and the worst, cam going "flat"!! Good Luck!