back fires around 1/2 throttle

I just put my new 327 in my 89 coupe, the motor is brand new, sportsman block, GT40X, heads, ported, and pocket ported, 284hr comp cam, holley systemax intake and 1" spacer 70mm throttle body, and egr spacer, 80mm pro-m mass air, 30lb injectors, long tbe headers 1-5/8's,  the motor idles fine runs fine until i get past 3/8 to 1/2 throttle, it back fires and no power, i checked fuel pressure when it does this it stays at 40lbs, I checked for vacuum leaks didn't find any, the TP sensor, is at .97 idle 5.0 wide open, it opening it stays smooth on the voltage, I bought the mass air used, is it possible the mass air may be bad?  

Possible? Yes, but likely you'll find something else. I'll look into it and get back with you.  

Double check your timing, distributor, wires, plugs. as an experienced mechanic, i've performed tune ups and thought something major was wrong, to find a crossed wire, or off a tooth on the dist. Does this occur in park, or under a load?