The two piece rotors were used with the two piston calipers, from 72 to 76. In 77 they went to a one piece rotor. As Scabber said the offset is different, the early have more off set to fit the two piston caliper. The later have less offset to use with a single piston caliper.
I don't think adding a spacer behind the rotor will work, wrong way to move the rotor. Adding a spacer between the caliper and fork won't either, moves the caliper too close to the wheel.
You need to get an 77 up one piece rotor.
Leo