Ricky Racer,
I'm pretty sure almost any Honda (as well as other Japanese brands) shifter arm will fit on the CB750 shifter splines.
I went to the Honda dealer and asked if I could look in their parts for a shorter shifter arm. They let me look in their bins, and I ended up using a Z-50(minibike) shifter, mounted backwards for an upside-down shift pattern. It was very cheap.
I would take your shifter off and to the parts counter with you, along with a measurement of how long you need the new one to be, and ask the same thing. Maybe ask the parts guy if you can buy him a soda for his trouble, too.
I'm planning my rearsets for my current project, and for the brake side, the footpeg will mount just aft of the brake pedal pivot. I'll either shorten the brake pedal and the brake rod arm on the back side of the brake pedal shaft, or cut the brake pedal and drill it to use it as part of a linkage.
Sherm