How to Measure for the correct back spacing?

magnumdart said

We don't know your exact configurations, but there are a few things that can get in the way including exhaust pipes on occasion.

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I've never seen any vehicle that had the exhaust ran from inside the leaf spring, over the axle, and then to the outside of the leafspring? It would make no sense and more than likely it would cause interference with a stock wheel and tire.