Russ, if you have an M-Head without a motor, I'd highly recommend doing a vari-speed conversion on it -- you'll almost never have to change belts.
So get a 6-pole, 1125 RPM, C-face motor with 50 - 100% more horsepower than the stock motor, and put a single sheave on the drive side that mates with the middle RPM, so when you over-clock or underclock the VFD, you'll get the full range of spindle speeds.
This approach also has the huge advantage that you don't have to extend the motor shaft (since you don't need the entire cone sheave). The stock Bridgeport motors are custom pancake motors with extra long shafts.
"Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."