28 by 9 MT slick, 15 by 8 rim with a 4.5 bs is what I recall with my old stock 10 bolt setup. It was real close to the outter lip. A 5.5" bs would have hit the inner framerail. Again, this is the MT slick. Though looking at the 275/60... ET Street Radial, it has a section width of 11.0" where the 28 by 9 slick has a 10.8 section, so? .... you may need a spacer in addition to a BFH...
15 x 8 wheel with 5.5 backspacing and spacers. I rolled the outer lip and took a BFH to the inner wheelhouse, and both a 28 x 10.5 M/T slick and 275/60 drag radial fit fine. I have pictures at my fquick page in my sig below.
PS: You can even get a 295/65/15 radial on an 8 inch rim under, but it's a tight fit!
My Nova had 28x9 M/T slicks on 15x8 wheels with 5" backspace.
Close on the inside and out. Fendewell lip was rolled and the inside had to be massaged some. The car also had a stock 12 bolt in it. I think the 12 bolts about 3/8" of an inch wider than a 10 bolt in these cars. Rancho rear shocks in the stock location too.