1964 Chevelle - need help with backspacing & fitment

[robs64]

Hey all, I have my wheels picked out but I assume I need to know the proper backspacing and wheel fitment so that I don't spend a lot of money on wheels that look good but don't fit my ride.

As I mentioned the ride is a 1964 Chevelle. My current set up (as it was when I bought the vehicle) is more for the drag-strip then pro-touring. I have pizza cutters on the front (15 x 6 maybe) and much larger meats on the rear 15 x 10.

I want to drop in 18 x 8 on the front and 20 x 10 on the rear and plan on running maybe a 230-240 series tire. I eventually plan on running the ridetech GM A Body - Level 1 package. Also FYI the guy I bought the car from removed the front inner fender wells. I have not necessarily planned on putting replacements in there as I live in AZ and it rains all of twice a year. In other words I am not too concerned with the width spacing in front as I am in the rear. If I do replace the inner fenders it will likely be a custom fitment built around the wider/custom rims.

Question is what method is used to determine what backspacing will work? Is there a way to measure this? Would Discount Tire or other similar style tire shop be able to pin point what I need? Any help is appreciated.

[65 drop top]

20x10 in the rear is gonna be very tight especially if its going to be low. Most 20x10's I've seen have trimmed the wheel well lip and some modify the inner fender well. If you look up in the wells you'll see that they get narrower as you go up, meaning the lower you go the tighter they'll be.