The block is probably the same, the ignition might be different, the lower unit like you said is different: I think in 77 they went to a one piece housing for the bigger motors.
Post pics of both motors?
Jerryjerry is right about the LU, they started to use a one piece LU in 1977- 78. In 1975 some of the 135Hp was equiped with the MagnaPower II igniton until 1977.
Then they started to use the Prestolite ignition system.
Everything else except the LU and maybe the ignition system are swappable. I have a 1975 135Hp myself that I am going to restore and it has the MagnaPower I system with a belt driven distributor.