I am running a Phoenix Models P-51 Strega with a TT PRO .61 and an Ultra Thrust Pipe.  It is competitive in Silver on 10% fuel.  I ran it in Bronze with the stock muffler and you have to slow down your times or you will break out.  The Strega is banned in SAMS Warbird racing but the motor should be competitive in the airframe you have decided on.  The ST .65 I know nothing about but the guys running Bronze at Kingman last month were doing just fine with a TT PRO .46 and stock  muffler in the Strega.  The ST .65 with the stock muffler and on 10% might break out of bronze if you fly a half way decent course so that means if you go wide or high on a few turns you still might be competitive.  Anyway that is where I would start.  Remember don't cut any pylons, don't jump the start, and finish all you heats and you could place at the end of the day.