Couple Problems..... Soft Code???

[BowTie Eater 5 Liter]

well ever since i put my motor together the fawker hasnt ran 100%.

what ive done is edlebrock aluminums, 1.7rr's, e-303, cobra intake, underdrives, full exhaust, sn-95 alt, 70mm t-body, 73mm c&l, afpr, 190lph pump(stock 19lbers unfortunately).

my problem is after 5000rpm the car doesnt like to work, it backfires and gets really stoopid... i just changed o2's, fuel filter, fuel pump relay(was getting a code for that). i get a "soft code" if i tromp on it through the first 3 gears(or more). the check light will come on and stay on until i let off, and i can not pull a code up for it. i was thinking the o2's were the problem, but now im leaning towards mass air sensor??? i havnt found a way yet to actually test to see if the meter itsself is screwed? does this sound like a condition of a bunk mass air meter?

another q, what should i have the fuel pressure set to running my combo(as ive just got the stock injectors for now) or should i leave it at 39psi?

thanks