I have a 79 2500 pickup with a 305 (holley 600, edelbrock performer intake, 400 trans) When I crank up and try to backup (after warming up) the engine sputters, and spits on the hill out of my garage. any ideas of what would cause this?

My guess is a flat cam if it has a lot of miles on it or bad fuel.
I'm told 305s are notorious for their cams going flat.
I'm putting a new one in my 305 Chevy pickup now and the old one was real worn.
It popped and back-fired and had no power whatsoever.
There are a myriad of things other than the above. Just look in any automotive repair manuel under trouble shooting and you'll see what I mean.
Good Luck!

the motor only has 500 miles on it. Rebuilt from the block up

Double check your timing.

Check the intake manifold for cracks.
You'll have to clean the dirt and crap from it first.
My pickup backfired and it also had a cracked intake around the carb.
Tighten your carb to make sure it isn't sucking air around the gasket.
Good luck.