The seat in my '66 is approx 21-22" wide as it's widest point of the seat bottom. I didn't measure the seat track spacing, figuring it was wider than the II's.
Another seat to consider is a set out of a pinto. I'm on a mission for a pair of hi-backs similiar to those used in the '69 deluxe and '70 std. interior mustangs. Some Pintos came with these and i assume they'll fit just the same. Will find out soon.