My rear 17x8's have a backspacing of 5.25" and I need to do a bit of inner fender work (mainly because on quarter panel and inner fender were replaced had it has a big lip.) with 245/45/17's. 5.5" would have been more ideal as I have some room to go in. I too have a factory A-body 8-3/4 rear converted to LBP. Your best bet is to measure you car and on both sides. Up front I runs a 17x7 with a 4.75" back spacing.
Now on my soap box. Ditch 15" (and 16") wheels. Tire options are getting less and less and the compounds, tread design and tire designs are 20-30 years outdated. Sizes are limited and getting very pricey. I'm not saying get some 24's and hydros but 17" wheels make tire selection a hell of a lot easier on all accounts. When it came time for my car I couldn't find much that was a direct fit in any size then when it came to tires I wasn't happy with Cooper Cobras or BFG Radials and then saw the price and limited sizes....
My car below:
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