I hope someone out there has some good ideas because I am running out. I have an 83 Dodge Ramcharger with a 318. I am running into a misfire and backfire through the carb under heavy acceleration and heavy brake torquing in gear. The timing gears and chain have been checked, the timing has been set, compression is between 125 and 140 on all cylinders. #5 cylinder seemed slow to build compression but came right up when oil was added. #5 plug looks like it is running lean. The heads have been milled and the valves and guides replaced. I have swapped out the distributor and a carb from a known good running 318 (from my brother's Duster). What else could cause a backfire and misfire under heavy load?? It seems like a lean misfire but don't know what else to check.  

Check the dist. cap, wires, coil, and plugs.
Make sure ign. box is good.
Check the fuel pressure, low fuel can and will cause a misfire under load because the engine leans out.
Also check the fuel filter,
I would double check the low cyl. it should be within 10% of the other cyls. on a dry compression test. If it isn't you need to find out why its low.