I've done the reverse on a friends bike. That is, fit a '78 grab bar to a '76. It fits to the bike the same but I seem to remember that the tail light mounts are different i.e. your 78 tail light might not mount up on the 75 grab bar. I forget if it's just the backing plate on the tail light or the mounting plate welded on the grab bar that's different but if that 75 bar comes with a tail light I guess it wouldn't matter.
I've got a '78 grab bar that's covered in copious amounts of rust that you could have for cost of shipping (Oakland,CA). Also have one that's in pretty decent shape that I could let ya have cheap, if that helps.