I just had to read this and comment when the poster asked for a NEW T-45...
There could be a couple new units sitting in warehouses out there somewhere. I've never encountered one, but I wouldn't want to install one because they would not have ANY of the updated parts so they would not be reliable. Also, they may have rust on the inside as happens to transmissions that are stored without climate control. Additionally, there are three versions of the T-45. 96-98 GT, 96-98 COBRA, and 99-01 GT/COBRA. I will add I've never seen an 01 COBRA with a T-45; I think they went ahead and used the TR3650 for all of the 01 COBRAS.. So, I'm saying there is only a 33% chance a T-45 someone comes across would even fit.
So, here is some "History" of the T-45 upgrades:
With all the updates, a well-built T-45 is very reliable and smooth shifting. There are 26 spline input shafts available to increase the launch torque it can hold up to, but the quality of those parts is all over the board, as those readily available are imported from Asia... The T-45 SHARES the synchro hubs and synchro rings for gears 1-4 with the classic T-56, but the T-45 gears are not quite as strong and there are no optional gear ratios available for a T-45. You can't even change the 0.68 fifth gear ratio to something better for racing..
Tremec took over the T-45, T-56, and T-5 production from Borg Warner around 1996. Ford was rightly upset with Borg Warner over the poor reliability of the T-45, so, for 1999, and some late production 1998 units, all 3 T-45 forks were made stronger, the 1-2 and 3-4 shift plates were made a little stronger as well. Those 99+ style shift plates have the letter "T" stamped in them. There was even a Ford TSB regarding special 5-R Forks. Unfortunately, NONE of those beefed up parts were made strong enough to hold up. They ALL still broke almost as much as before. Furthermore, for some crazy reason the new style 3-4 forks were made thinner where the shift plate fits which allowed the shift plate to act as a "slide hammer" and the shift plates beat on the upgraded forks causing them to split and fail even more than before.
Rebuilders say they put the "updates" in their units, but their definitions of "updates" are all over the board. Mostly, they use the carbon-fiber synchro rings, which is all that can be bought today anyway, and round up the 99-01 style forks and plates from old transmissions and claim they are updated. Some do use custom billet forks for 1-2 and 3-4, but few also replace the shift plates with the thicker, custom shift plates that are made out of a stronger alloy of steel than the originals. These are much better than the plates with the "T" on them.
It was Keisler Engineering who specialized in the T-45 addressed many of these issues with most of the custom parts I mention, but they went out of business several years ago. There are at least two companies making the billet 1-2 and 3-4 forks similar to what Keisler was making. I only know of one company making the strong billet shift plates and the billet 5/R forks and I can tell you that those items are not widely used even though they are critical upgrades.
There are other little upgrades for the T-45 as well. All this is to say be VERY CAREFUL in thinking someone's rebuilt T-45 is updated to solve all of the T-45 issues when they say it has been updated. Custom parts are pricey, so a great T-45 build will not be inexpensive.
Stan