Since I had only owned Beetlebomb for a couple of months, and had not driven him much, I was worried about running him in out 4th of July parade and not knowing if he would overheat.

I had also read about heat getting into the passenger compartment even when the heat is shut off.

I also saw a posting about a PV owner adding a brake duct fan to his heater system for more oomph.

I picked up a small muffin fan at Radio Shack - 12 volt, 85 cfm at .3 amps.

I removed the hose from the heater box and mounted the fan BACKWARDS on the box. I connected the fan to switched 12 volts so that the fan would run all the time. I left the hood unlatched to allow some extra heat to escape.

Wow, extra cooling and NO heat in the passenger compartment.

My brick did get a bit warm - three miles of stop and go over a one hour period - a lot of other antique cars dropped out, but I think that the fan helped quite a bit.

BTW - the bilge fans on ebay look a LOT like the duct fan I saw here, and they come in 3" or 4" round, up to 300 cfm.
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'96 855R, '64 PV544, 95 855, 854 - 385,000 miles put on 8 bricks.