18" wheels

1971 CY Dodge Dart said
I'm running Ridler 18x8 wheels over my Wilwood brakes. The tires are 235/40-18"s. I bought the car with these and the 18" have grown on me.
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My son is looking at 18” Ridler 606’s for his swinger. Do you know the backspacing on these? Also could you go with a little fatter rear tire?