Back Spacing

I don't know as though there's a standard contact point to measure from. Generally if you measure to what appears to be the nearest obstacle you'll be close enough. Front and rear have different obstacles. Subtract 1.5" to 2" for clearance, tire bulge, etc and you'll be safe. You could get away with less but your measurements have to be more accurate.

Unless you order a wheel with a custom offset, they're going to have common backspace dimensions: ex: 3.75", 4.0", 4.5", 5", etc.

Many late model wheels have high positive offset (a lot of backspace). Gotta be careful with these. Short of taking measurements, I'll bet you could run an 8" wheel - front or rear - with 4.5" of BS. A 7" wheel about 3.75"

Try putting a jack under the control arm so that the suspension is loaded and not hanging down all the way. Put a piece of tape on the floor. Then, use a plumb bob (or make one) - hang it from the fact of the hub to the floor and make a mark on the tape. Hang the plumb from the inner fender or spring or whatever and make another mark. That'll give you the distance. Or place a straight edge against the face of the hub and use a ruler to measure from the straight edge to inner fender.

Go to AmericanRacing.com and check out the tech pages or newstalgiawheel.com

Hope this makes some sense.