Mainly used on bigger V8 HD GM vehicles and commercials in the 70's.
Typical more common use in 72-78 El Camino, Chevelle, Monte, GM c10- C30 heavy duty along with many motor homes and GMs.. really gotta match what your got.. IF.. IF what you have was really original.
4 bolt, large center 2.5 inch..thats about 64mm
19.5 inch OD. which is a migen under 50cm.
Be carefull with the OD as many fans also only take up to 19" or less.. fitting a 19.5" blade will make ticket-tape of your shroud.
Make sure your engine mounts are rock solid or enough to allow a bit of clearance on reving if your fans inside a shroud.