It'll fit if you raise the rear end up a couple of inches(or more). If you aren't into that look, then 275 is probably the biggest you can go before cutting out the tubs. I like my stuff lowered, so I mini-tubbed it. Now the ears on the rear end touches the trunk floor before a 315/60-15 touches anything else.
Im running a Hoosier drag radial P295/65R15 on an 8" rim with no frame rail mods on my 85 Monte SS. the tire is a 29.5" dia with a 10" thread width and a 11.2 " section width and I must say it is pretty close.