Bruce,
   At this point, the T-10 Borg Warner tranny, used extensively in early Shelbys, is nothing more than a door stop.  Parts availability is poor to non-existent.  The transmission is notorious for popping out of gear, and nobody seems to be able to cure that problem even though Shelby owners and others have spent serious bucks trying.  I have owned a daily driver T-10 Falcon and road raced a T-10 Mustang (briefly).  IMHO, you could not give me a T-10.  That being said, there will be more than one fellow who claims he can indeed repair the tranny. 
Best,
Al.

From what I recall the T-10 was basically replaced by the Toploader in performance applications.  GM also had used the T-10 in various offerings, but replaced it with an in-house transmissions line when they started offering the M20 and M21 Muncie, and eventually the M22 for serious performance use.  I think nothing's changed.  The OEMs saw fit to replace the T-10 with their own transmissions.  Now one could argue that this was motivated more by cost than anything having been wrong with the T-10, but it also begs the question why didn't either Ford or GM just buy Borg/Warner or the license for the T-10?  From a strength standpoint the T-10 was a pretty decent trans, and eventually BW came out with the Super T-10 (an aluminum case 4-speed that bears a striking resemblance to a Muncie) that was GM's go to from '73 until the end of the 4-speed manual trans and the introduction of the first 5 speeds and other overdrive transmissions.

IMO if you're building the car to drive I'd get an overdrive trans in it and go with a T-5 or Tremec.  If its got to be stock, rebuild parts for the T-10 are available, but hard parts can be tough to find.  A Toploader would be a better choice if originality doesn't matter and you want a 4-speed. 

Toploader for sure.  No question it is the strongest standard tranny Ford has offered ever.  It is virtually indestructible and rebuild parts are available.
Al

I have a Toploader taking up space in my garage and I would be more than willing to part with it. It came with a project car I purchased. It was rebuilt by a guy who specializes in Toploaders and was not installed since rebuild. It also has a new McLeod clutch, and pressure plate, plus scattershield. It was for an engine that was expected to make between 450-500HP. I wanted a daily driver with overdrive so it is collecting dust. FYI.
SteveinNC

It is a wide ratio tranny based on the numbers. The output shaft has 28 splines, the tailshaft is 14", length of the main case plus the tailshaft is 24". The tag on the case reads "HEH B1 015466" numbers on the main case "C5AR-7006-D" and tailshaft numbers are "C4ZR-7A040-A". I have not come across the paperwork yet, but I know I have it somewhere in a box with a bunch of other receipts. Probably hid that box from my wife and forgot where I put it. SteveinNC