There may be a filter just inside the inlet of the carb, changing bodies could have got some junk in it. It probably isn't a vacuum leak concidering it ran good before. It still could be contamination in the float bowl as swapping the motor around could have stirred up some junk.
If it does have a healthy cam in it 15" would be ok.
The engine angle is a good point as well, I didn't think of that.
Another thing could be the accelerator pump, if it sat for a while the pump could have dried up. Easy way to check that is to look down the throat of the carb while the engine is NOT running and hit the throttle. If both squirters are shooting out an even stream then that is not your problem.