Chris,
I believe it depends on the year and model of the car the motor came out of. I assume you mean a 289/302 smallblock. A 351W may complicate this even more. I have seen 2 different water pump and crank pulley diameters, and if you add PS or A/C, things really get crazy. And, there are two different alternator pulleys as well, a lo-po and a hi-po (which came on Shelby Mustangs, etc.). If you are looking for a starting point, find someone who has a stock late 60's Mustang and measure. Or, caontact March and they can give you diameters for their pulley kits.
Good luck-Jack
