Backfires when put in to drive

Hello, I got a 1988 silverado c1500 5.7 liter automatic. The engine was rebuilt 3 years ago guy said they put a cam in it for towing. It ran fine until about 2 months ago. It started running rough and couldn't get up to speed. So I parked it and tuned it up. Replaced plugs, wires, cap, rotor, fuel filter, and airfilter. Then it wouldn't start. I change the fuel pump, coil, throttle body (with a used one that work just fine). It cranks over just fine checked spark and it was orange was told that it was to be blue. But I noticed that the injector were dumpin fuel in the intake instead of sprayin or I add a bit just to see what would happen it fired up and ran like a champ. After about 30 mins of idling I put it in drive and it backfired like crazy and stalled. It would start right back up but then it started to backfire while idling and then quit but still backfires like crazy in drive. Was woundering if a weak spark would cause this. Or maybe jump a tooth of the timing chain. I can go out side right now after a week of sitting and it fires right up with a couple of back fires and is idles smooth. Better then it did better because I tuned it up but it never back fired before. Maybe it could be a messed up cam i'm lost need some help please thank alots dustin