Hello, may I suggest that you compare offsets, and not backspacing?
The offsets of most Explorer wheels should be very close, just like those of Mustangs or Cobras.
The offset is easily measured. It is half of the difference between the back spacing and the front spacing.
Offset=(BS-FS)/2
It is easier to use the offset figure, because it is based on the centerline of the wheel. Knowing the original offset, you can easily calculate in and out based on the difference of a new prospective wheel.
I have 96 Cobra 17x8's with 30mm of offset. I liked the fit and I am also about to need new tires. I looked at 18x8.5's, but wheel dealers wanted me to buy a 20mm or less offset. That 10mm+ of outward distance, plus 1/4" of a wider wheel, would have my wheels sticking out at least 16mm farther than tyhey are now. My next option was a 35mm offset, and that would require a spacer of about 1/4". Those wheels were not available now, so I will probably just get tires. I did get 1/4" spacers to possibly help with the slightly wider tires that I am looking at.
What I learned about my wheels and their offset allowed me to make good decisions about prospective wheels. If I relied on the "experts" of some wheel dealer, then the wheels would look the way they wanted them to, not my way.
Decide how you want your wheels to fit, find your offset information, then find wheels or make what you have fit the way you want it to fit.
Good luck,
DW