Looks like in 78 and up they put a restriction in the pipes, the 77's dont have it, it was meant to improve fuel economy, if you cut them off and grind them to full open again it, then reweld them, it should gain you some HP, you will have to rejet after this mod though.......
or if you can find a 77 one use that, the pic below shows a 77 one ont the bottom left, and the 78up one on the bottom right.