10" seems to be the most popular width for the Coupes. I have 15"x10"s on my XB, with 5" of backspace. It's close; can just get a fingertip between the sidewall and the fender lip (unrolled).
US owner Reed Herman's (former) XB Coupe was fitted with 17"x 11" rears
. IIRC, he once posted his biggest worry was the occasional rub on the INSIDE, against the rear leafs.
More on his car here: 1975 Australian Ford Falcon - Featured Vehicles - Car Craft Magazine
(it's since been sold, now in NZ)