As Steve mentioned, I did trim the x member in my 55. Only an expert tbird person could ever tell it was done I think. Looks like a factory thing.
Anyway, so that would give let you put an AOD in there. I used the Bendtson adapter and kit which incudes a high torque mini starter flywheel and adapters.
It will (I think) work also with a scrub R700 trans by using a different flywheel and crankshaft adapter. I discussed which trans to install with Bob Bendsten if he were doing his own Tbird and he recommended the AOD.
I kept my old Fordomatic and all assorted hardware so that if the person who gets my car after I'm dead- can return it to it's old ineffiecent, heavy self.
After owning this car all these years I can't ever see going back to stock now or worrying about the half inch I shaved off the x-member that no one would know about if I hadn't told. ; )
I also was able to retain the stock shifter which WAS A MUST for me and have full selection with it.
Here's a pic of how I did it.
A L A N F R A K E S ~ Tulsa, OK