My '72 Paramount P-15 has 27" Weinmann rims. Not sure about the original tires, but 27" X 1 1/4" Pasela gumwall tires work fine. The Weinmann rims are straight-sided, so you can't pump them up to more than 70 lbs pressure without risking a blowout. I forgot about this, so I know from bitter experience
Modern rims are usually hooked sided, so you can pump them up to 100-120 lbs if you like.