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.