As far as I am aware , the K rear fender will fit.
The seat will prob. not be a direct fit, due to different style fuel tanks.
Drop CBJUNKIE a PM, I think he was messing around with something similar to this...
Yes, ProTeal, you are right about CbJunkie. I remember the postings about that. I live near a motorcycle salvage yard that has literally thousands of Japanese motorcycles and was trying to give him what little info I could. While going through the motorcycle graveyard, I saw differences about the mounting locations and configurations of the seats on the various years and models of 750's. All I wanted then was the attaching metric sized pins with the holes in the end that the "locking" clip goes through. I never found out how CbJunkie finished up his project.
As far as the rear fender goes, I found in that wrecking yard that there are slight mounting differences among the years and models. I have a 1974 CB750 K4 and at another point in time I had to replace the most rear piece of my two piece rear fender. In order to bolt to the frame, I found in my case, that only a 1973 and 1974 rear piece would match up to my frame bolt holes. Fortunately, I did find a 1974 rear piece in very good shape that matched the mounting of my 2 piece rear fender. Perhaps some earlier years would match, but I found that later years would not match what my bike had. (Just for trivia info, that rear fender I picked up had excellent chrome, no scratches, no dents, etc., etc.,......... I had to pay a whopping $10

).
As has been stated, check with CbJunkie and find out what he learned. I did not keep a complete "notebook", if you will, about what years would interchange and what would not. I mostly ignored what would not work and stopped my "research" when I found what would fit.......
I wish you good luck.......
~ ~ ~ jaknight ~ ~ ~