I can't say for certain, but I don't believe that there would be any difference in the engine mounts. Are you goin to use the '76 swingarm too? I believe the 77-78 is different and trying to swap the later SA to the '76 frame could be an issue.
The seat lock is different on the 77-78 and I don't think the later seat will fit the '76 frame without some mods.