UGH. Your going to want the rear axle flange to axle flange 54 5/8 with a 6" backspace on a 12" wheel. This will error to the narrow side so if anything you can use a thin shim to dial in exact wheel location. It is easier to move them out then move them in! Standard disclaimer: Of coarse the best thing is to mock the wheels in place and measure between the hubs, but with that said, we done many of them this way.
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Frank Serafine
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