You should also be aware when looking at used parts that there are a number of versions of the T56. I believe that there is eight. As for the 5 speeds that came in the earlier Mustangs: Some years had T-5 (5.0 era), some had T45, some of the newer are TR-3650 (360 lbs-ft) and I believe certain Cobra models have a TKO version but not sure. So again if buying used know what your getting because a T5 will not handle what a T56 or TKO does. As chicane said above these transmissions will require adaptation ie; Crossmember, driveline length, shifter location just to name a few. There is a lateral-g member that had T56 for sale. Here is link
https://lateral-g.net/forums/showthread.php4?t=10742
Also another member that has LS1/T56 combo for sale
https://lateral-g.net/forums/showthread.php4?t=10627
I believe chicane works with Tyler at ATS (not sure) so if he does him or Tyler could help you with T56 conversions. It is what they do and do it well. They are a sponsor here.
http://www.americantouringspecialties.com/main.html