Are you sure that is a 'T' on the one with the heavier flange and ribs? Although they certainly beefed them up later on, I thought that didn't start until the TR5/TR250 introduction, when the prefix changed to 'CD'. I run an even later 'CC' prefix housing, which I chose to get the beefier casting.
If it is a "CT" prefix, then you should be able to move the OD without drama. I'd want to check the clearances and so on during the process (since you'll have it almost completely disassembled anyway); plus a pressure check on the OD before installing it in the car.
However, when I installed the OD onto my CC series box, I wound up having to use the complete gear set from an early box. I forget the arcane details, but the CC gearset wouldn't match with the CT mainshaft I had.
In my case, the starter drive had come apart while I was on a long drive. The loose drive components eventually got jammed and knocked a hole in the bellhousing; which started a crack that ran all around the flange, ruining the housing. I wanted the beefier housing as extra insurance against it happening again (although I've never had another starter drive come apart like that).
You probably know this, but JIC, there are some adjustments you'll need to make to fit the 4-synchro box. It's about 3/8" longer than the 3 synchro, so you need to slot the holes in the rear crossmember to move the mount back to match. I also had to bend down the flange on the back of the crossmember, to keep it from interfering with the mount. If you use the box with the thicker flange, you'll also need to use longer studs on the back of the engine, and modify the clutch slave slightly. I chose to keep the slave on the back of the flange and just make a shorter pushrod; but in retrospect it might have been better to pound the flywheel cover flat and move the slave mount to the front of the flange (on top of the cover).
It wasn't a problem on the 3A, but on my current TR3, I also had to enlarge the hole where the speedo cable attaches to the OD.
Make sure you get the right carrier and throw-out bearing to match whichever clutch you use. The CT54xxx box would be from a TR4A, which used the later setup.
I much prefer the TR3 shift lever; you may need to modify the top cover slightly to accept it. Or what I did was to take an early TR3 top cover and install the TR6 shift rods and forks into it, so I could keep the dipstick. But your car won't have the dipstick anyway, so it's probably easier to modify the later top cover.
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