There is actually a lot of room in those tubs in stock form.
I want to say a 295/50 R15 will fit inside without rubbing if you get the back spacing right on the rim, 10" rim of course.
That's a 26.5" tire (aprox)with just over a foot of section width and over 10" of face contact to the pavement.
It doesn't leave you a bunch of room but your leaf spring pack is your inboard boundary and of course your outboard boundary is the lip where the quarter meets the outer wheel house.
The inner wheel house is damn near straight up so you'll never rub on the inner with a big tire, the outer wheel house half tapers in so you could rub there with a big tire and a trunk full of deer or something.
