Well, original 15x8 snowflakes have 4.5" backspacing, so getting Cragars with 4.5" backspacing would be good so they would fit like the original wheels do. And the most popular size tire on 15x8's has been 255-60-15's. I looked at the backspacing specs on the Cragars and it appears that 4.25" is as close as they have. That would probably be okay, since I don't think 1/4" difference would cause a problem.
Reverse offset means that more of the wheel sticks outward than normal, so the wheel has a deeper look. If you look at the "Rear spacing" spec on a Reverse offset wheel, you'll see the backspacing is only 2.5", so that means more wheel is to the outside of the wheel center. So, since the wheel sticks out further, you could have tire interference with the body wheel opening lips.