From 74 up all the necks were the same length. So all forks with the trees from 74 up swap.
The 74 -76 were 34 mm forks and used the two piston calipers. The 77 up were 35 mm forks and used the single piston calipers.
The 76 used a two piece disc brake rotor, The 79 used a single piece disc brake rotor. The two piece disc has more offset to work with the two piston caliper. About .2 inch.
So putting the 79 forks on your 76 you need to use a 77 up disc brake rotor. These work with the single piston caliper.
Leo