Far be it for me to question a knowledgeable vendor, but that just doesn't sound right. I don't think you'd even get 8" of backspacing to clear the lower a-arm.
The stock 10" front and 13" rear GT5 Campi wheels (basically the same 13" wheels as quoted) are 4.625" front and 4.75" rear backspacing (to inner lip of tire bead).
Julian
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