Jeff, the only experience I have with a 91 AC was in a pattern airplane and I ran Powermaster 20/20 through it with a 15X8. I think guys are propping all the YS engines to spin between 10K and 11K. I can't see where the RPM numbers would be different with the 91.

As far as the airplane goes, if you think it will stay together then I say go for it. Unless you are going to mod it to closely resemble a Reno P-51 that did not have a belly pan then I'm pretty sure you are going to have to keep it. I would also keep the wing area. There comes a point where less wing area is slower in the straits because of the added AOA needed to support the weight of the airplane. IMO 6 lbs with less then 475 sq in is in that zone. The added wing area will actually get the airplane through the turns faster and there are 20 turns per race. Loosing 1/2 second per turn puts you down 10 seconds or 1.5 laps. It would be great to have you out there but having an airplane that runs with these guys right out the gate is a very tall order. Of course I will be the first one to shake your hand should you pull it off.