Hi Gang,
That's right Ed. Even the long-stroke is mis-applied with a muffler, but it runs great with a 13-7 APC sport prop!
The "half pipe" designs like the Jett and Ultrathrust should run fine.
Half pipe is a misnomer, these high performance mufflers were designed originally for the Q-40 "muffled" racing class.
The tuned length is folded, in a manner not unlike the old Phelan Magic Muffler, Rossi Q-40 muffler and others.
They work great, and I believe that their manufacturers will sell you a 90-sized muffler, or an exhaust stack extension,
for a 60 that is intended to be run at moderate RPM, in order to satisfy noise considerations.

later Friends,
Dean