Welcome to the Group!
There has been a lot written over the years about transmission swaps, so keep using that search feature (and others will reply here) for further self-education.
Some people use a 4 speed manual, most a 5 speed. There are specific transmission models recommended. Some people with a factory 4 speed manual add an Overdrive unit. Some people just swap the rear differential to drop the RPMs. Some people swap out the whole rear axle for one from a 280 sedan, as they have a 3.27 diff, but then you have to do brake work as you would be changing rear brakes from drum to disk.
The 4-speed auto behaves like a 3-speed because the stump-pulling first gear is only engaged by the kick-down switch on the floorboard, or by putting the gear selector in "2". I never had a complaint about my auto in my 230SL. But, I don't mind the high RPMs relative to modern cars, or even US domestic cars of the 60's. My 230SL has the 3.75 rear axle ratio, so that helps some!