I'd try a 7 pound cap first ... you don't want to put too much pressure on an older system; it could cause bigger problems.

My truck was boiling over when I first got it ... I found out it had NO thermostat in it! I put a 180 thermostat in (I have a 350 sbc) and all was well.

~~ Alan Horvath
1954 Chevy Pickup
Singing his praises in the
Passing Lane