The only reason to run a pressure cap is to raise the boiling point of the coolant. Every 1 PSI of pressure raises the boiling point 3 degrees, so unless the coolant is getting above 212 degrees for some reason no pressure should be needed. Just install an overflow tank and don't fill the radiator all the way to the top. Any coolant that ends up in the puke tank will get drawn back into the radiator, as long as there's a good seal between the zero-pressure cap and the top of the radiator filler neck. You can make a zero-pressure cap by removing the lower rubber gasket of the existing cap.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!