Topic: backspacing

[66stang]

I want to get some 18x8.5 wheels on my 66 and I was wondering if anybody knew what backspacing I would need to make those work

[Soaring]

That's not going to happen on that '66.  The largest wheels listed in Dodgestangs chart is 18X8 in the rear with 245-40-18 tires, and 17X7 in the front with 205-40-17 tires.  And, the owner had to trim out the inner fenders.  And, the BS is 4.5 in the rear and 4.25 in the front.  Not sure you can easily find that big of a wheel with that small BS.  If you use spacers, you are going to rub the fenders everytime you hit a bump. 
http://www.dodgestang.com/Tire%20Fitment%20Guide%2065-6.pdf