The advertized back spacing is 5.72" for the Mach1 wheels, you will need to add the thickness of the inner bead lip to that which puts it near 5-7/8".
Edit: Sorry, I read your post wrong, I just measured from the outside of the wheel lip to the edge of the rim last week and it is close to 1". So if you took the 8" wide wheel minus the 5.72" back spacing, that would be 2.28" + the bead lip thickness close to 1/8", added to the inch that would be around 3-7/16". This is for the front, the rear you would add about another inch.
Haha, must have been the way I worded it. I'm trying to figure out if I bought my car with a drop on it. I want a measurement from the middle of the center cap to the fender/wheel-well on a mach with stock rims/tires.