FWIW, you can use a 1985-1993 Ford Taurus electric fan. One 15" fan does 4500cfm. You can get 2 of them to fit on our radiators (at least a 2nd gen).
The Lincoln Mark viii? fans do 5500cfm but they are either 17" or 18".
I know this is a very popular mod for high HP gassers.
You can also get BRAND NEW taurus fans for under $100 each, and summit sells a dual fan wiring kit with termostat. For under $300 you can have double the airflow of the Flexalite.