Ford 390 backfires through Carb on acceleration and under loads, HELP?

What year is this 390? How many miles are on it? Also, has it been pinging prior to this issue? Also does this motor still have points in the Distributor? There is a very likely possibility that you may have a Warped, bent, or burnt Valve. If timing is correct, and your have made a positive check of everything else. Then the only other solution is the fire is getting out of the hole. If you have a compression tester, I would run a compression check on all the holes and see if there is one that is low or is not holding compression. This will be your problem hole. We had a 72 Buick Le Saber 455 in the Shop about 6 months ago that was exhibiting this same issue. The guy had changed everything out right down to even converting the ignition system over to HEI. With a compression test we discovered the #4 hole had no compression. We used an inspection scope to look in to the cylinder and discovered that the valve was burned. We pulled both heads, did a valve job on the problem cylinder, and the motor runs great now. I really hope this is not your problem as it is a lot of work. But it is a possibility.