Some are custom offset some are Mustang as they have the same 5x4.5" bolt pattern as Mopar and also have the 5" or so offset we need. I don't know which years Mustang had the 5x4.5" bolt pattern and correct offset.
I went with Billet Specialties custom offset because I needed a 5.5" backspace on my 66 Coronet to get the most rubber I could in the rear with no tubbing. None of the production rims are available in that offset (as you know) and I didn't find any Mustang wheels I liked.
Beware if you go with 5.5" offset as a 17" or smaller rim may not fit due to hitting the upper ball joint. I went with 18x8" to be safe. Tires are Nitto 555 255/45/18