My own personal opinion is that you would be better off with a good mechanical fan, either with a clutch and a fixed 7 blade fan or an OE 7 Blade Ford Flex fan (they just flat move more air than the aftermarket flex fans) AND A FAN SHROUD. This will insure that the air is actually moving thru the radiator. It will be a little hard to find, but the 64s with factory AC would have had a shroud. A factory shroud off something latter model should be fairly easy to adapt if originality is not a problem.

The 7 pound cap will not do you any favors, it will blow off sooner resulting in quicker boil over if it does overheat. The only reason to run a low poundage cap is if the cooling system (primarily radiator and heater core) is weak and your afraid of them springing leaks.

The 6 cyl and 289 radiators were a bit on the smallish side to begin with, you might go with an oversize aluminum radiator (with shroud) like I did in my 64. I got my radiator off E Bay for less than $200 including shipping. That, a factory flex fan and a shroud were more than enough to keep my fresh engine (with a .110 overbore) cool during breaking yesterday. 180 degrees with a 180 degree thermostat.

There's some pictures of my setup (to include installation) here if you haven't already seen them

http://www.clubhotrod.com/forums/sho...t=16968&page=3