Yeah, a T-5 will go in with very little trouble, and the adaptors are
readily available. I'm putting one in my kids' '65 as soon as I can
round up the parts. But other than the fact they both have an input
shaft and an output shaft, a T-56 has no similarity whatsoever to a
T-5.

Here's a website about getting a Tremec 3550 into a '65
http://mustangsandmore.50megs.com/Pages/Tremec.html

Check this photo for an idea of how much sheetmetal needed to come out:
http://mustangsandmore.50megs.com/Pages/Tremec014.jpg

A T56 is bigger than a Tremec.

180 Out