Here are the dimensions of a Hayden #2797 that I put on my bird. It fits a
5/8 fan shaft.
Hayden also has a 3/4 shaft version P/N 2747 that is bored for a 3/4 fan shaft and comes with a sleeve to reduce it if required.
The fan clutches slip - even when they are fully engaged - so be sure you use a big enough fan with enough blades to cool things when you need it.

This is a Pontiac 7 blade aluminum from 1968 - there are many around to chose from.
Steve Metzger Tucson, Arizona