Found my dads original 1971 duster!

Awesome. Thanks! I’m not sure the builder wants to use it. I’m also not sure how to tell if the clutch would be any good. Looks like I’ll be buying new. I guess at this point, what another $300 for a new fan. Lol
If its not leaking and there is resistance when when you turn the portion that holds the fan while holding the attaching flange.
Some clutches had a thermal control on the front - that one probably does not. best part is that it will fit and be spaced properly without spending a bunch of time figuring out and checking stuff.

The difference between clutch and fixed fan is measurable. In terms of power, the difference between two clutch fan set ups is going to be small. I'm surprsised the builder is so resistant. Clean the fan up and use it. Soda blast or something gentle if you need to blast.