I can't tell you for sure.... but that sounds about right.
I have a pair of factory-made centerlines that look like they were made for your exact situation. 10 1/2x15, 7 1/4 backspace, 5 on 4 pattern. They have been double drilled to 5 on 4 1/2. They are basically a 15x7 front half, 15x14 back half.
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