need help getting the 350 up n running

[69heavychevy]

my friend and i got through installing a brand new gm crate motor on his 72. but for some reason we cant get it to run right. when he puts in drive and hits the pedal it sputter and backfire through the carb. but i ease onto the pedal it will accelerate fine till 20mph or so and then start backfiring. we've checked and double checked wires, vacuum hoses, timing, and just about everything else. like i said he has a brand new gm crate motor (260hp i believe) 1-5/8 long tube headers (open, no exhaust), edelbrock performer rpm, edelbrock 1406 carb, msd street fire dizzy set to 8 degrees, th400 with posi 3.08. in park we can mash the pedal all day with no sputtering or backfire, but as soon as it goes into gear it starts acting up. any idea of what the problem could, thanks in advance for your help.