Hello Stephen
Summit has a Ford Racing T-5 rebuild kit for US$255.95
Ford Racing T-5 Transmission Rebuild Kits: FMS-M-7000-A - summitracing.com
They also have whole boxes (Ford Racing 'Super Duty') for US$1400 which might be worth considering depending on how severe the problems with your box are.
Good luck with the Tranny - going to make ShelbyFest?
LoBelly