Pissed off with motor, want to change

I have had issues with backfiring through the exhaust. I installed a new cam and it ran beautifully all day. I took it to the car wash, drove in running fine, washed the fender wells, started up backfiring. Never dried out, so I changed plug wires. No dice. Then plugs, cap and rotor. No dice. Coil. Uh-uh. Ignition module. No. I am thinking the car wash was a coincidence and I have a flat lobe or bent valve.

I was going to go with a 408 stroker this winter so I could enjoy driving all summer. Now I think I am going to push the timetable up. I need to find a 360 block. There are a few around locally pretty cheap. Does it matter if it comes out of a truck or a motorhome? I don't think so, but I don't have the knowledge base this forum has! I found a running 360 and transmission out of a motorhome for $600 and a non running motor out of a motorhome for $200 less than 15 miles from my house.

Much as a new stroker appeals to me, I would rather wait. Any advice on the popping through the exhaust would be great. I can get it to stop popping for the most part by adjusting the timing, at least until I put it in gear. It's driving me nuts.