HELP - Backfire/Popping thru Exhaust !!!!!

[6600DURAMAX]

Finally got the 454 off the test stand and dropped in the 72, and now the problems start. When the motor was on the test stand (about a month ago) everything seemed to run fine (ran it for about 1/2 hour or so). After I dropped it in and got everything hooked up the motor fired right up and sounded fine, ideled good, great throttle response and so on but.........once I take the motor to about 2500 rpm it begins to backfire/pop thru the exhaust.

Thought it was timing - re-dropped distributor in twice, adjusted the timing everywhere on the scale

Thought it was tight valves - readjusted all the vavles

NOTHING HELPED... do not know where to go from here.......

The only thing different from when the motor was on the test stand is the MSD Box (6AL)??? Could that be bad it is only 6 months old and ran fine when I pulled it off the sbc.

The motor is a used motor that I bought a while ago that is why we put it on the stand and fired it so we wouldn't have these issues

Any suggestion are greatly appriciated

thanks guys