car won't crank.

A lot of work swapping starters in a small block. Better to do some diagnostics. Clip a multimeter to measure the voltage between the big stud on the starter and its case. If it holds >8 V while cranking, then you can question the starter. Otherwise, try to find the drop in the wiring. It may even be on the return side, so check starter case to BATT- while cranking. Could be the ground from engine block to BATT-. Don't assume anything, test all paths. I once had the dash cluster pulled out in my M-B and noticed smoke from the speedometer cable while cranking. That clued me to a corroded ground cable connection from transmission to body, which a little sanding and silicone grease fixed.

Also, a good time to check compression. Some camshafts can give excessive cranking compression which can fight the starter.