I am in the process of rebuilding a 66 Mustang Fastback. What I would like to do is put a 5 speed in to this instead of going back to the original 3 speed that I have. Can someone tell me what vehicles used the T-5 and what all I need to get to complete this conversion.

Thanks for any help

The T-5 will work as long as you change or modify the crossmember and get the driveshaft changed.  It'll probably be shorter than stock but there's drivershaft shops that can shorten yours or make a new one for you.  Can't say if the shifter hole would be different or not since the T-5's longer than the 3 speed.