13", 8" wide rear wheel, what rear wing be suitable? There are different widths, I gather. Thanks.

It will also depend on how wide your axle is, and the offset of the wheels...

Yep, as humbug says. I actually measured mine (hub face to plumb line from arch) and then worked out what width wheel required what offset to remain inside the arch.

I'm reasonably certain most people only use one of two size arches, narrow or wide which I think were developed for escort and siera width cars respectively.
Dax and probably other do use wider arches still.

Ned.