To fit 275/60's in my '68 Dart sedan, I needed 15x8 wheels with a 4.75" BS. That was with a rearend built to A-body factory width. It's best to get your rearend in place and measure from there. With a 1/2" spring offset, there's not a lot of margin for error when it comes to backspace if you intend to run a 275/60/15 tire. It cane be done, but it's a little tight.
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