Help Diagnosing a 302 that backfires through the carb only under load, sometimes stalls.

I recently finished putting together a 302, I replaced the head gaskets, cam bearings/cam. My problem is the engine backfires through the carb only under load. I'll start by saying I think mechanically the engine is ok, my results of a compression test were between 150/155psi on all 8 cylinders. The backfire is one poof through the carb, not repetitively like a worn cam lobe. The block is an 85 that was previously TBI but now has an Edelbrock 1405 carb on a ford stock aluminum manifold. The ignition is an aftermarket Chevy style HEI distributor with the ford bottom. I verified spark on all 8 cylinders, I triple checked my firing order, and one of my questions is will a 302 run on both standard or HO firing order. The cam is barely a mild one just a few thou over stock, nothing radical. I'm not sure what's causing this it also sounds like it has a slight misfire, but revving it in neutral it seems to run fine until I let the clutch out, then pop and it stalls. Any help is greatly appreciated.

I'm thinking the cam would be the difference in the firing order difference. I do see that the difference in firing is on opposing cylinders where the pistons should be at the same positions. When my frustration calms down, I'm going to try switching the wires to see if it will run and if it makes any difference.