The auto parts store had a 620 wheel cylinder and it fit! The threads on the center (hollow) bolt fit, but the threads on the four mounting studs are different than the stock nuts. Stoffs I'd imagine you're right the 620 studs are metric and the 320 nuts are SEA. I'm going to take it to hardware store, find the right nuts, and should be set!
Also I replaced the coil, hooked up the battery negative ground and she runs! We had a blast driving it around the neighborhood figuring our the shift pattern. I'm shocked at how peppy she is and 4th gear is a dream!
Next up, finish the breaks, install a new thermostat and coolant overflow, and drive her around some more! Oh and maybe bolt down the seat and install seat belts.