This thread has morphed between a 2001 GT and 1993 Cadex. The 2001 is probably a 100mm, and yes a 2001 Judy TT could also be 80mm.
A 1993 Rockshox can’t be 80mm or 100mm. My response above was for the 1993 Cadex, although it does apply to the GT, except the brakes are linear/v-brakes, and not cantilever, so the levers have a different cable pull.
I would guess there are mechanical disc calipers that will work with linear levers. I only bring up mechanical as I don’t know how well a hydraulic disc front/mechanical linear brake rear would feel in practice.
John