basically any side mounted seat that isn't the 'large' version will fit for the S body and Skyline ones, I would assume this is the same for the AE86 too
the measurements we have for the rails and seats aren't the same measurements
The rail measurement is for the exact base width that will fit inbetween the rails, whereas the seat measurements are typically the total width of the seat rather than just the width of the base of the shell.
Ken has one with a normal width Border seat fitted in his AE86 for reference.
Basically, if it's not the XL or large versions, you should be fine