I would expect the diff to be a different ratio on the 320. The diff also varies with 4-speed / 5-speed auto. There are a few ratios to choose from so break out the calculator and work out what you want for rpm / mph for your usage
If you go for a TT setup you are into lots of work with fuelling, compression ratio, etc
Personally, if I were going to go to all this trouble I'd find a wrecked 500SL and build something unique
Nick Froome
the independent Mercedes Estate specialists