just rebuilt runs bad

Did the shop degree the cam at all? Sounds very similar to the 360 I just put together. I had installed the timing chain one tooth off. I have no idea how I managed that, but the engine turned over kind of hard on break in but fired up fairly quick and was able to run for a full 30 min and sounded decent at 2000-2500rpm. At idle my mild cam (edelbrock performer rpm cam) sounded alot more choppy than it should have. When I tried to drive it it had almost no power. I had to feather the gas and even then half the time it would backfire and die.
After looking over everything I could think of-carb, ignition, rocker adjustment, etc. I drained the antifreeze and oil and popped the front cover off and saw the chain a tooth off.
Hope this isn't your problem, but its something else to consider checking out and at least its fairly easy and quick to do.