'75 750F has a rear disk brake? Use a modern sport bike rearset pair. They have the rear brake mastercylinder mounted as part of the rearset. Generally they have the shift linkage also, so reversing the shift pattern isn't necessary. I use Yamaha FZR600 rearsets on all of my vintage bikes. Nice small compact unit. Available on Ebay for less than $50 a set. If you have a rear drum, then it's possible to weld a piece onto the rear brake mechanism that you can hook the brake rod to. You need to pull the rear hub apart and reverse the brake mechanism so that the rod pulls on the top of the hub as opposed to below the axle.
TR