Ford electric fans from a Lincoln Mark VIII appears to be the highest cfm fan. It is a 2 speed fan so it runs at a lower RPM & only runs on high speed when needed. As I recall it is 18" in diameter so a shroud would have to be made. The V8 Mustangs are the same except for the shrouding.

I have heard that Walker uses this fan motor with their shrouding.

CFM's ratings on after market fans will not be seen in real life conditions, so I would use the highest CFM fan that will fit.

IMHO I would try to find a fan rated around 2700 cfm.