Fastest piston packer ever - in case you're interested - is Lyle Shelton's Bearcat "Rare Bear" which achieved something over 528mph at Las Vegas in 1989. Mind you, it's got some serious airframe clean ups and there is an R3350 lurking under the cowling which turns a modified P-3 prop I believe!
Mustang "Dago Red" - now retired apparently - has a 15km closed circuit record of 517mph, so might have been a contended to mount a challenge....