A NOS T44, a real one and I don't think there are any would be worth at least $50,000. Lee Holman can provide an exact replica for about $25,000. I own a four speed T44 and an auto T44 but they are spares for J6 and aren't for sale.
RBT can provide a transaxel that can take a big block as can Quaiffe.
MK IV J6: I always wondered how the setup of the auto trannny was: since T44 was basically 9" rear end casted together with Top loader, how was the auto tranny done? We ´know it was a short lived attempt, and also was part of trials that killed Ken Miles (due to maybe cracked brake disc.)
My auto T44 seems to have the same Weisman Rear and case but it's internals are different. When I bought J6 this transmition was included with the spares I got. It seems to have been used at some point but in what car I don't know. Lee Holman is the expert on these boxes and if you email him he may be able to answer your questions.