no power at all in gear

[Fast68Chevy]

305 engine, runs fine runs good revs up fine all until you put truck in gear and try to get truck moving from dead stop. ive tried everything including swapping on older model distrioutor thats non computer controlled and same with carb too and swapped on good timing chain set and it still does exact same thing, try to get it going in gear and engine will bog down and truck wont hardly move at all under its own power and will eventually backfire up through carb and makes poofing out exhaust, but doesnt do all this when is not in gear. i know the 2.73 gears in the truck dont help acceleration and all but still it should be able to get going without popping and missing all over the place out the intake and exhaust both,

any ideas ? this is a new problem ive never ran into before on these, intake was swapped form 2bbl to 4bbl in its past maye they just did something wrong when they swapped it, i dunno,. maybe dizzy is 180 off, but if it was i dont think it could run as well as it does out of gear.


thanks for anything