Mount locations for shocks and spacing of foot peg mounts on frame and the rear fender as well as the turn signals and the width of frame/seat all are radically different on cb750 vs cb550/500.
Width of bikes makes a difference when you see that most panniers for hard cases tie one side into other with tube that connects near taillight or behind license plate.
So, if you have the originals and fab up a mockup of thebends and twists with 1/4" copper tubing, easy to form, you can use it to make patterns or as a guide when bending or welding up brackets to adapt your rack mounts to the bike.
Then you have to find someone who is a good mig welder or tig welder if making new frame from hard aluminum rod or square tubing.
Then if dealing with Chrome tubing for frame you have to re-chrome or stop rust from eating the modified frame...
The Shoei bags and frames for a 550 are out there but the cult like following will cause you to pay dearly for them.
If you go with soft bags you need to build or install things to keep bags out of the rear wheel and chain and stabilize the bags.
If you want funky, find a vintage set of Vetter hard saddlebags.
You can harvest the mount points off the Goldwing theytypically come off of after building a frame to bolt them to so you can build your replacement mount frame for the 550/500 frame.
BTW, swingarms for 550F and frame for 77-78bikes are different ... So, don't be disappointed if you find aset from a 550k from 77-78 and then they do not fit your 74-76 k. Shock locations are identical but, foot peg mount method changed, don't know if they moved the position.
