I doubt it. Every shop I talk to says the offset for mustangs is +24mm so 40 might be too much. Don't quote me but it sounds like they won't clear your brakes.
Good question.
What really matters is backspacing, the amount the rim sticks towards the center of the vehicle. Offset is just the difference from centerline toawrds the outside of the vehicle (you can have positve of negative), so if the center line is half of the OA width...7"/2 = 3.50", and the offset is 40mm (1.57"), then the true back spacing is 3.50" + 1.57" = 5.07"...which is plenty good, max on the back is about 6.5" backspacing and I run 6.11" on the front.
So I think it will be no problem! Hope this helps!