I'm going to put forest flare arches on my road going Mk2 Escort.

The wheels I have have the number 723 15x7.5JJ stamped into them.. but I'm not sure if that tells me the offset or not?

In order for the wheels to fit the arches nicely, and not look too narrow, what width should my rear diff be from face to face (so the part the wheel bolts too)?

Would those rims be ok? or should I order a different size?

Thanks