I like to use SearchTempest.com for stuff like this. It will search all Craigslist in your area, and EBay at the same time. For the fun of it I found several Holley 750s near me for under $200.. I think you would be happy with one with vacuum secondaries and electric choke; these are easy to adjust with different jets, secondary springs and such... use the blue gaskets, they are reusable a few times.
Looking at a carb calculator for 512 ci, a Holley 750 or 780 cfm carb will suit your needs for a street motor, until you get near 6000 rpm. How often will your motor see that...