Slow progress, but getting there. To start, I've had to find another brake lever with a shape that doesn't arc down then up, like the one in the photo posted in Nov last year, which touches the top exhaust megga and doesn't allow enough room for adjustment.
I also decided to have a go at converting a 1965 CB160 rear brake plate, which has a cable mount. The two plates are pictured, the drum size is the same at 180mm.
It will need drilled out to 20mm to fit the 750 rear axle. The inner plate for both 750 and 160 fits against the wheel bearing, and the width to the outer on the 750 is 45mm. The 160 is narrower, only 28mm. So another spacer 17mm will need to fit to bring it up to 750 specs.
The inner 'shrouding' on the 750 plate is fractionally greater diameter than the 750's inner hub, at 71mm. On the 160 it's only 59mm in diameter so the entire shroud needs to be milled out to approx 72mm, and just deep enough to allow the inner brake plate, where it is slightly raised, to sit against the wheel bearing. That is, a slightly raised section where the axle fits through.
Finally, there is a double 'flange' around the outside of the 160's plate. To get it to fit snug, the inside part of this flange needs to be milled down 1-2mm for it's entire circumference.
All this should work. I'll let you know !