The problem is not the price but finding a used T56 that will fit a mustang, Camaro or Viper T56s wont, they are too long. Most Mustang T56s are "aftermarket" specially made for the mustang.
BEst bet is eBay.....or try with Hanlon motorsports or DDperformance, both migh have some used or rebuilt T56s.
The swap involves installing the right
trans xmember, driveshaft, pedals, yoke, cable, clutch, p-plate & flywheel..thats about it.
DDperformance.com i beleive has the list of parts (also sells the kits).