You did not get the 78 wheel too? The disks are different (different part numbers between 75F and 78F), and I believe the hubs are different too, to accommodate the extra disk. If you like the spokes, you can get adapters to mount a comstar hub to a spoked rim:
http://motosynthesis.blogspot.com/p/products.htmlNot sure about 750 compatibility. The write up on this page is for a CX500.
I don't think the 78 750F calipers are designed to work with 1975 rotors. I think the 78F rotors have a different offset, and I think they might be thinner too. If you got the forks and calipers for the 78F then I would recommend getting the rest of the system:
>Front wheel, with rotors
>Brake lines
>Master cylinder
Edit: In the comments section it is indicated that these work with the SOHC CB750 style Comstars