Do you have a selection of 17" rims/tyres to try on?
I'd try to fit a 17x8 AU onwards rim, to see how much space you have inside the well, and a 17x8 pre AU rim, to make sure it fits on the outside of the well, then, if there is a little more room, add it in to the combined size.
Just making figures up here, but a 8" rim is 9" wide, with the bead added in. Add a tyre, maybe 1 more inch. So a 17x8 should be about 10" wide sidewall to sidewall. So your 10" rim will be 12" wide all up.
If a pre AU rim has 4.5" bs (dead center) and an AU onwards rim is 6" BS, then you have 1.5" difference.
So if a AU rim fits, with an easy 1/2" more room on the inside, and a Pre AU rim fits OK on the outer, you've found the extra 2" you need. So you would need a 6.5" BS 10" rim, to be the same.
Backspacing and front spacing, MUST equal 1" more than the advertised rim size. So a 8" rim, with 4.5" BS, MUST have approx 4.5" front spacing.