Thanks again guys. I was so happy to find a real Sanden NOS R12 single pulley compressor...
@forestg, it's still a 5 piston compressor and mounting spacing is the same but I think you're right though, I believe it does have more displacement than a 508 (164 vs. 131cc) and might not be an issue. Let's see, 25% more volume per rotation, but 75% less turns per RPM. There's probably an elegant equation for all of this.
@davidnunn, you're making me wonder if this pulley is designed for a larger belt - it's a wide groove, maybe this thing's for a tractor or something?