wheel ?

[mbeach]

4.5 or 4.75" backspacing,
4.25" hub bore. This is 108mm.

If you start talking metric you'll confuse the salesman.

EDIT:
Factory wheel (18") backspacing was 5" (a 7" rim with +38 offset).

An 8" wheel (any diameter) with an offset of +0 will have a backspacing of 4".
An 8" wheel (any diameter) with an offset of +10 will have a backspacing of 4.4"
An 8" wheel (any diameter) with an offset of +20 will have a backspacing of 4.75"

I'd stick with the 4.75" offset in order to minimize rubbing and trimming/bashing.