Crazy Backfiring

[roysgreenstang]

Ok, Ive had my car for 5 years. Last year i put some money into the top end of the engine. I got a pair of CGM 60cc combustion chamber 200 cc intake runner heads with 2.05 valve double roller rocker hardend pushrods, gear driven timing gears, an msd blaster ignition coil, a 90 mm lincoln marc 3 maf with the 24 lb injectors that its calibrated to, a cold air intake, full length long tube headers, flowmasters, and a b 303 cam. I put all of this on after working out every kink i could find i still have a major backfiring problem under any, and all amounts of load. How ever under no load it seems to be running fine. Except for the terrible 5.0 surg that i also cant figure out. I have platium plug, that are the correct plugs for the heads, that i have changed several times, just to be sure, i changed my wires, cap, rotor, distributor, took out the salt aad pepper shaker wiring harness extention pigtails, checked all the grounds, insured that the calibration of the tps was on at 1 ohm. Even took the computer and put it on another mustang with similar upgrades and everything checked out. I am at a loss here. I did hone the cylinders before i put the engine together and a couple of my cylinders are not exactly on par with the rest, most are at 150 psi, which im not sure what that is for compression ratio, and 2 maybe 3 are at either 135-140 psi. Could that be causing this. The backfire is so bad that i cant drive it at all. It is backfiring through the exhaust, and possibly through the intake, though i cant be too sure of that. I was using the stock intake manifold, throttle body and fuel delivery system, i had 20-22 inches of mercury vaccuum, the valve timing was at 0 and the ignition timing was at 13 degrees advanced base timing, however i tried messing with the ignition timing, to no aveil. I have had ever mustang guy i know look at it, and it stumps them all, i was thinking of getting a good block and starting over with that, any advice on this situation would be highly appreciated. I also got the typhoon intake and a 70 mm throttle body now so if that helps please send advice, i would like to drive it this year. Another year without her would just be torture. Thanks