Here's the simplified version Sam, should be as clear as mud.
The P2 class (my bike) is restricted to twins (max 750cc 4 stroke / 500cc 2 stroke), manufactured up to the end of 72; wheels 15" or greater, max 5" tread. The XS qualifies as a continuation of the TX650. The best motor for the class is the BMW because of it's low CG. It's drawback is the shaft drive, and expensive parts.
The P3 class is for short bikes with smaller then 15" wheels (eg 13" or 10" like Bear's Windle), and wider than 5" tread. The Fiddaman uses 7 & 8 x 13 slicks. The engine has to be a 4 stroke, production based, available up to December 31 / 82. It use to be the engine had to have an
original displacement of less than 900cc's. The SOHC CB750, DOHC CB750F, Triumph Trident, and Kawi 750 were all eligible engines, and you could bore/stroke them to the class max of 900cc's (+5% over bore). The CB900F (901), and Kawi Z1 (906) were not allowed because their original cc's exceeded 900, not including the over bore. The CB900F is now eligible, along with the Kawi, so is the CBX six.
As awesome as the six would sound, it's probably a poor choice because of it's width, weight, and cost for racing parts. While probably not the 'best choice', I've got x2 SOHC 750 engines, and an MR 1060 kit to work with, so that's what I'll use. Being a dry sump has both oiling, and weight distribution advantages as well. Using a 900 crank adds stroke, and brings along the Hy-vo primary chain, both good. As I said, I'm going to have to think & plan this out while I restore the chassis.