I run 18x9's and 18x10's with 275/35/18's and 295/40/18's on my Duster. But making that work and taking advantage of the Falken Azeni's with a 200 tread wear takes more than just the right wheel specs. Obviously it takes that too, but literally the only stock suspension parts I run are the spindles.
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It sounds like you're building a "pro touring" style car, and depending on what other suspension work you wanted to do you could run 18x9's with 275/35/18's all the way around if you did a 1/2" offset on the rear springs. Which would be pretty simple if you're already having a rear axle built.
I'm guessing you measured that on the car with a tape measure? If anything they'd be the same or narrower than the BBP 7.25's, which themselves are narrower than the SBP 8 3/4. I haven't personally measured one of the '73+ SBP 7.25's, only the BBP version, but based on working with other members here on their backspacing all of the 7.25's are narrower than the 8 3/4's. The backspace that works on an 8 3/4 puts the tires right on the springs with a 7.25 of either bolt pattern variety.