confused on backspacing
Does not matter if the wheels are 14", 15", or 17", because wheel diameter has no bearing on backspacing at all.
This is not accurate whatsoever. On a 14" or 15" wheel you can't run more than about 4.75" of backspacing in the front because of contact with the UCA, regardless of width. That basically limits you to a 7" rim in the front. Yes, if you work it out just right you can use a 15x8", but there isn't much point because you can't run a tire wide enough to matter. You're basically stuck with a 235, and that will fit on a 7" wide rim.
If you step up to a 17" rim, you can run as much as about 5.6" of backspace, the rim clears the UCA (with a low profile zerk on the upper ball joint). Most rims start to interfere with the outer tie rod around 5.6" or 5.7" of backspace if you're using 73+ disks as the reference. Some can run a little more, but not most. 5.6" of backspace lets you run an 8" wide rim with ease, and you can run 245/45/17's or even 255/45/17's without issue or interference.
If you go to an 18" rim, you clear the outer tie rod. An 8" rim can have as much as 6.3" of backspace, again using the 73+ disks as the reference. A 9" rim will now fit up front, usually with the best fit around 6" to 6.2" of backspace. You do have to watch the UCA as full lock with that wide of a rim, tubular UCA's come in handy to run an 18x9 or 18x9.5. But now you can run 275/35/18's in the front, like I do on my Duster.
I think where you'll see the metric dimensions is for "offset". A 7" wheel with 4.50" backspacing has 1" of positive offset or around +25mm. An 8" wheel with 5.00" backspacing also has 1" or +25mm offset.
These numbers are wrong too. A 7" wheel with 4.5" of backspacing has an offset of +13. An 8" wheel with 5" of backspacing also has an offset of +13. Backspace and offset are not measured from the same places.
For the OP, do you know the part # of the wilwood kit? There's more than one, and the offset added by the brakes is different with different kits. What kind of tire width are you looking for? Backspace requirements change with the width of the wheel. Do you want to keep a 225 wide tire up front and a 245 out back and just go to a 17" rim? You could run 245's all the way around pretty easily, although that's probably about as wide a tire as you can fit in the back with the stock spring locations.