4433, the max allowable tire width on a given rim depends entirely on how you will use your car. If you simply enjoy the thing at relatively low speeds on your local streets and roads, run about any size you or your tire mounter can manage to get on a rim and hold air. But if you 'vigorously' drive on mountain roads, open track events, high speed cruises or autocrosses, anything wider than about 275 on an 8" rim will contribute to sloppy handing that can put you into a guard rail, even with 45-48 psi of air inside. And while that much pressure will help the sloppy handling- a little- with oversized tires on narrow rims, it will also ride rough enough to cause your fillings to swap teeth on bumpy roads.... Generally, 295 or 305s will need a 10" rim, and some competition sanctioning bodies mandate allowable tire vs rim sizes.
FWIW, Hoosier offers a Z-rated 275-50 x 15 tire that fits an 8" Campy, and H-rated tires are still readily available; while H-rating is 'only' 130 mph, that means 130 continuously for 30 full minutes- which will likely get you shot by your highway patrol in most States! It doesn't necessarily mean an H rated tire will instantly explode if you momentarily blast to maybe 150 while 'warming up the engine'. At Bonneville, H-rated tires are legal for 150-mph runs since the run-duration is only maybe 5 minutes & the salt is cool. So it depends... as I said, whatcha gonna do with the car & where you gonna do it?
