Mark,
My street tires are Michelin X's on stock 4.5" steel wheels, in 165/70R13, and I have plenty of clearance. My race tires, on the 5" wides, like Jeff, are 175/60R13's and they wouldn't clear without a bit of grinding on the fender lip (and a panhard rod). However, this grinding is really minimal, and I expect that the 175/70's probably a bit narrower at its widest part.
Unfortunately, the Kumho's are reputedly .6" wider than the Yokohamas, so there's little chance I can make that work without completely removing the fender lip, etc.
P.S. If anyone has a wasted set of Kumho's, I'd be interested in taking them (and pay shipping) so I could eventually test fit them. They don't need to do much of anything but hold air (and have the same shape as a new one). Seeing as this is the time of the season, perhaps someone will have some around they'd like to be rid of.
-- Oh yes, another reason that the 175's will work on stock rims is because the 5" wheels have the same backspace (the extra .5" sticks out toward the fender, so the fender clearance is less than it would be with a stock wheel by roughly this amount). Since I have space on the inside of the wheel, its likely that there would be no tire-spring contact either.