The transmission swap will only net you a decrease of about 3-400 RPMS on the highway. And that's assuming it's not already an early-model trans. The difference is in the fill and drain plug and 5th and reverse switches. You can't trust the shifters. I can't remember which carries which, though. I believe the early model fill plug is a square 3/8 hole and the late model has something like a 22mm hex head. Don't quote me, though. But if it's the Sammi I'm thinking about, it's probably already an early trans. That was a no-expense spared build.
Ultimately, though, you're really operating that motor in its sweet spot. VW diesels tend to like higher RPMs. The weight gain and increased drag from bigger tires will probably offset anything you'd gain in MPG. Might make it worse. It's got a broad RPM range, moreso than most diesels. You should be getting pretty good MPG with your current setup. I'm guessing you'll see between 25 and 32 mpg. don't expect VW mpg, though. You're pushing a lot more air with a Samurai. It's got the aerodynamics of a washing machine with the lid up. Plus, it's lifted, so that just adds to drag. It should do better than a stock Samurai, but don't look for 45mpg.
