Pulleys are really tough to figure out. Just remember each fit a different size shaft and saginaw are keyed. The first Federal pump use was 67 with the 318 C body, so no big block stuff yet. Saginaw pulleys do have part numbers on them in many case which helps identify application.

For your small TRW the Big block pulley is taller when on a bench flat. The gap between the body of the pump and the pulley is larger. Small block TRW pulleys ride close to the pump. Also, remember there are belt thickness issues to watch as you mix and match pulleys.