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The seller reports they're 18 x 8.5, +30 offset, oem 2011 bullitt wheels. With 245-40/18 front and 265-40/18 rear, is the diameter difference sufficient for the computer. Will they stick out too much?
Well, if that's the true dimension, they'd work as they're exactly the same dimension as OEM Z rear wheels. The tire sizes work but the 265 is a bit outside the maximum range but not by much so they'll fit with a small amount of tire slop.
BUT, key here: check to make sure the hub hole is adequately sized (66.1mm or larger and can use with suitably sized hub rings).
I checked here and don't see anything close to that size listed but maybe the Bullitt wheels just aren't listed. (Nor do I see 70.3 ==>66.1 hub rings available) https://www.wheel-size.com/size/ford/mustang/2011/
But, after all that, if you just gotta have 'em, measure first!