Hey mate,
First of all, congrats on the price and build!!! I too have a 77 cb 550k that i purchased for 250 bucks and had some of the similar problems with it. It had been sitting in a garage since 1986. From firsthand experience, ALOT of the 750 parts from partsnmore will interchange. I have successfully ordered and installed both front and rear brakes from the 750, shocks, starter relay, wheel spokes, turn signals, and a few other parts i can't remember right now. Anyway, as far as the front pads are concerned, order the round pads for the pre-77/78 and they are a direct replacement for your front pads. I made the mistake and ordered the 77/78 750 front pads, and they were too big. They exchanged them for the pre-77/78 pads and they worked. Hope this helps! I am trying to get pics of my build on here.. I've taken mine down to the frame and rebuilt her from the ground up. Should be ridin this spring!
Erik