I've been impressed by the tire clearance on older racing bikes from the 70s. My '72 Fuji Finest and '77 Motobecane Grand Record can take very large volume tires. I have them currently set up with 27 x 1 and 1/4 and there's lot of room for a fatter tire. Plus I could run 700c wheels and gain a little more space for a larger volume tire.