You will likely need around 5.75" back spacing on a 9" width, and might consider 18" dia depending on what brakes you want to run. There companies that offer rallys with custom backspacing, which is what I'd recommend for what you are wanting to do.
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69 RS LS3/T56
68 firebird vert LS1/4L60E
67 RS LS2/T56
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