Don't forget those plates on the Z mount on the swingarm pivot bolt and engine mount. you will have to make spacers up to replace or shorten and rethread those 2 bolts.
I made spacers (see pic), but it looks like your footpegs are mounted to those plates while mine (82 650sc) are mounted to the engine mount bolt.
Is your brake pedal mounted to that plate also?
Take a look at raask rearsets for a cb750 k7 for an idea. They use a curved plate that attaches to the swingarm and engine bolt.
You might also consider a cable to operate the brake instead of a rod & pivot arrangement. I think I've seen the cable operated rear brake system as a kit. May have been for a sportster. Edit: see dime city cycles conversion kit.
Maybe the shorter plates from an '82sc could be retrofitted ( made shorter) to hold the rider footpegs where you want them. That plate is somewhat hollow, so a 1/2" aluminum plate shaped like a raask plate might be easier
