The head pipes will interface in the same way. So, no problems there.
But, I think the muffler hanger positions are somewhat different. As were the muffler hangar brackets.
Something else to watch for is the swingarm bumper position and muffler relief indentations.
The 77 used completely different carbs from the 75, so there may even be some jetting issues.
If you are still interested, I might be convinced to go out back and take some measurements on the two bikes.