What year vehicle are you considering the TREMEC for and what transmisssion is currently in the vehicle? TREMEC was the OE supplier for many of the Mustangs and still supplies a TR-6060 six speed for the Shelby. I know they have aftermarket T-56 Magnums to fit just about any Mustang.
The factory trans is a TR-3650. I think tremem makes a TR-6060 for our application so you should be able to order one from them. The GT500s came with the 6060 but if you find one from a different car, you'll need to change the bell housing and input shaft I believe. I am by no means a transmission guru though. Call tremec and see about it.
If your looking to go with the tr-6060 and have the tr-3650 now then your going to need a new 26 spline clutch to match the 26 spline input shaft on the 6060. The things you will need are the bell housing, tranny, Shelby short throw shifter or other aftermarket shifter that works with the 6060, 26 spline clutch, and a 1 piece driveshaft.