This is a picture with a 18 x 12 wheel - 6.75 backspace running a 335/35r18 tire. I think a 12.5" wide wheel would stick out. The inner fender clearance is tight with the 12" wide wheel and 6.75 BS.
I wouldn't want it to stick out or in any further with the 12" rim. For a 12" wheel I'll recommend a 6.75" backspace, and depending on how far you want the wheel to stick out, 12" wide rim for almost flush, 11 rim" for inset 1.1" from the outer fender, or you could split the difference with a 11" wheel and do a 6.25 backspace and have .5" inset from fender and a larger clearance between the wheel and inner fender well.