Two things to consider with shocks/springs:
1- Travel length. From memory, I think the Cb500/550 shocks are about an inch shorter than the CB750.
2- Spring strength. These are rated in pounds per compression length.
The 77 CB750 is a 512 lb bike with a 360 lb. load capacity. The springs accommodate this load distributed front/rear at an unequal ratio.
The 72 Cb500 is a 403.5 lb bike with a 300 lb load capacity. The springs accommodate this load distributed front/rear at an unequal ratio.
With CB500 shocks you can expect your 750 to sit lower in the rear and bottom out more frequently on uneven pavement.
They will fit on the bike, though. Don't take any passengers, ride only on smooth roads, and jack the pre-load to max on the springs.
Cheers,