Exhaust Backfiring

Hi. Driving me crazy. Put 5000 mile on my 340 this summer driving to the Dragon Tail then home for four days then off on the Power Tour. Car ran fine. Put the car on a chasy dyno in Dallas and made 301 HP with it 96° outside.. A guy I was running with looked at the Graff and told me a few things I could do to improve the numbers. He looked at my plugs also. He said check my timing. It was a little Advanced. Checked it it was! It was 38°. Have a MSD distributor and a AL6. Checked the advance and it should have been 25° but was more like 29. Changed the bussing got the timing to 12° @ 750 and 3400 all in @ 2200. Put a PVC valve on. Ran great for a while. Then moved the fuel filter from inches from my carb to the fender. Then trouble started. Would start up fine then backfire out the exhaust. power down. Ran like crap. Had a 750 Edelbrock on it. For the last two years. Figured I moved some dirt into the carb. Cleaned it ran good to a point???? Got pissed at the carb and put my 650 Street Demon on. More exhaust backfiring. This is getting long. Look for part two.