Tire and Wheel Measuring

I have a 1974 Duster with a 66 B-Body 8 3/4 and I am attempting to measure for new tires and wheels. This is what I have done so far.

1. Used a plumb bob from the top of the inside rear quarter lip. I measured 4 inches to the drum mounting surface.
2. Measeured from the plumb bob to the spring and this is 11 inches.
3. Basically I have 11 total inches in the wheel well to play with.
4. So I came up with the following spreadsheet in which I calculated tire width and back space/front space %. Does it seem reasonable?
5. So I am thinking a 7.5" Rim with 5" of back spacing. I will be able to use a 255-60-15.

Does this seem reasonable? Thanks,

Rim7.5Back Space5Front Space2.5Back Space %66.67%Front Space %33.33%Tire Width10 Back Space Tire 6.67 Front Space Tire 3.33