Although my initial reason for posting these questions was to find a good production IRS that could be harvested from junk yards, I am now thinking that since the set up from the Mark is too wide, I might consider using it in my chassis I am building for one of my '53s. I am using a '99 Explorer rolling chassis but I don't care too much for the standard rear suspension. The other '53 is going to have a totally custom built chassis using the drive train from a '97 electronic all wheel drive Aerostar. Both these vehicles have 8.8" diffs so it would be easy enough to swap out the gears from my straight 8.8's.
It may well be that depending on finding a good steady source of the T-Bird/Cougar IRS set ups for the custom hot rod chassis is not a good idea in that they are highly sought for many different kind of projects including Triking Harleys and Goldwings. That might have to wait until people start wadding up MKS's and Taurus's.