Personally, I'd pull it apart myself. Leaks are usually just bad seals and/or plugged vents (although a distorted OD adapter plate is another possibility). There is a good series of articles on the Buckeye Triumphs site, and another on the VTR site. Even doing the OD isn't that hard (I just put mine back together last night, and should be doing a pressure test instead of playing on the computer).
Putting in a TR6 box is quite do-able (I have one myself), but it does involve some adaptation and modification.
The box is perhaps 3/8" longer overall, so the rear mount has to move backwards. Easiest way IMO is to elongate the holes in the rear crossmember (but I also had to pound down the folded edge for clearance).
Your gearbox tunnel won't have clearance for the starter bulge on the TR6 box, so you'll need to do some massaging there. Make sure you leave enough clearance, as it will communicate a surprising amount of noise to the cockpit if the tunnel touches the bulge.
Depending on which gearbox you get (TR6 had lots of minor variations), you may have to install longer studs in the back of the engine block. But they can't be too long, or it makes the gearbox hard to install. (Guess who cut them too long the first time, and had to trim them down already in the car
)
You'll probably notice the higher 1st gear ratio, but IMO that is a Good Thing.
If it is a late TR6 gearbox, the speedo drive ratio will be wrong. I don't recall offhand if you can use the TR3 gear, or have to find the early TR6 gear.
And of course you'll need the non-OD speedo cable.
The TR6 shift lever is different, of course, but you can make your TR3 lever work. You'll probably have to drill the top cover, though.
You'll also lose the dipstick, unless you rebuild your old cover with the TR6 components (which also solves the shift lever problem).
Clearance is a little tight between the shifter mechanism and the back of the hole in the tunnel, but it cleared on my cars. Definitely obvious (with the boot off) that it moved backwards, though.
OTOH, I'm sure there are any number of people on this list that would LOVE to trade you a good working non-OD box for your old, worn-out, non-working OD :laugh:
PS, if you are interested, I have the original box from TS13571L, that worked just fine when removed. I'll even deliver!
:cowboy: