3" Xpipe to 2.5" tail pipes?

I have a friend who races a '70 Lemans "3600lbs", complete interior with an 8pt cage. He was running 10.80's at 127 with 3inch exhaust to the bumper along with X-pipe. From the headers back he switched it all to 3.5 inch exhaust and the car then ran 10.60's at 131. Same brand muffler "Dynomax ultra-flos" and X-pipe"Dr.Gas" just a bigger size. Its a 455 around 650hp.

probably not much help, but I'm inclinded to think a full 3inch would better suit your needs.