Of the two pictures posted here we obviously have a clutch type fan on Jim's sweet '68 A/C car (with an A/C belt issue?)
and Amervo's direct driven FLEX fan. There is also
a direct drive rigid fan that the fins don't flatten at rpm. This would be the type I use myself. I started with a two inch spacer and have played around with that. Right now I've got a 1.5" spacer and it pulls quite a lot of air like that.