Might well depend on which version of TR6 gearbox you get. I'm running a later box (CC prefix), specifically because it has the flange & ribs beefed up. In my case, there was no question of not doing something to make it fit; the pushrod was still holding the clutch depressed even with the pushrod touching the lever. But it certainly won't hurt to try.
Here's the best shot of the TR6 setup I could find on short notice. On the TR3, the bracket mounts to the rear side of the gearbox flange and then the slave mounts to the front of the bracket. On the TR6, it's the other way around, the bracket mounts to the front of the gearbox flange and the slave mounts to the back of the bracket.
I didn't want to ruin my original pushrod, so I just made up a new one. Ordinary mild steel rod from the hardware store, same diameter as original (5/16" I think, but that was a long time ago). Fine threads up one end, then the other end rounded (more or less) by hand with a grinder. But you could achieve the same effect with the original. Replacements are readily available now.
The length of the return spring is kind of important, it needs to have enough tension to always pull the slave cylinder piston all the way home after every clutch use. If not, the point in the pedal travel where the clutch engages will drift up and down. Might not bother some folks, but it really bugs me. (Geo will probably be along in a bit and tell us he doesn't use a return spring, so evidently it doesn't bug him.) But, since the spring runs between the bracket and the clevis, and the bracket is moving closer to the clevis, the spring may not have enough tension to always pull the piston back. I'm actually running that way at the moment, as I neglected to move the tab over when I moved Frankenbox from the wrecked TR3A into my current TR3. And it still bugs me, just not enough for me to actually crawl back under there and fix it (yet).
The tab I made is just a short section of flat iron (again from Home Depot). From memory, about 1/2" wide, 1/8" thick and maybe 3/4" long or a bit longer. It doesn't need to be that thick, it's just what I had on hand. 5/16" hole centered about 1/2" from one end for the mounting bolt, then a 1/8" hole near a corner on the other end. And a bit of a bend in the middle, maybe 30 degrees or so, because it didn't add quite enough when it was flat.