i'm in the process of doing one on an ext cab right now. there are a bunch of different part numbers for basically the same carb. look for a mid to late 70s pinto or mustang II carb, something with about the same motor size as the mazda. i purchased one for a 140 ci 2.3l pinto motor.
also check out mazdatruckin.com, there is all kinds of forum topics on the 5200s and has a seperate section explaining a 5200 swap on a Mazda, check it out:
http://www.mazdatruckin.com/B2200/5200.html