Hi Gerald:
During my tenure at Fox, the 36X piston and sleeve was used interchangeably throughout the existing 36 line. That includes all the models you mention.
The difference between the C/L "Sport" 36 and the 36 RC was only the case and the crank length. The use of the standard 36X crank in the 36 RC was only for more space for the RC carburetor.
The primary advantage of the 36X BB was Duke felt it had more ability for revving after post-sale modifications by the end user. On the test stand, there was not a measurable (RPM) difference between a production NB or BB X.
As to Duke's reason for the short case 36 Sport: Duke was trying for the lightest possible C/L engine with good power-to-weight ratio and less nose length to reduce the gyro effect of the spinning propeller. Interestingly, it fell to some of my experiments with same to indicate that a sleeved crank bearing DID have a small drawback that made some form of rolling bearing at the rear of the main a slight advantage. This would show up after extracting all the power out of a Sport 36 (crank retimed, venturi opened, on pressure, high-nitro, etc) and after continual maneuvers. It would seem that during continual maneuvers (as is the case in combat), a slight amount of heat sag at the crank journal could be detected (in Duke's mind, anyway). The rear crank journal was already at the "almost too loose" stage already (where it would have excessive run out), so honing more relief into it was not an option.
Nonetheless, it fell to me to hand make a production example, and mount it to a very light combat plane (with large, stylized "FOX 36" emblazoned on the outboard wing, top and bottom), and put on unofficial "demo" flights here and there around the '72 Nats. I used a Demon on pacifier, and ready to fly (sans fuel), came in at about 14 oz as I recall. It was impressive, but by early in the week and I pulled it and popped one of my hopped up 36X BB's on it. (Couldn't resist.)
My X turned more heads and got more interest than the Fox 36, much to Duke's disdain.
Andre