... and spent 5 minutes doing a web search for Mercedes 300TD gear ratios & stock tyre diameter then ran the numbers. The Mercedes factory engineers gear the 617 engine to turn 3000 RPM at 65 MPH. That should be your target RPM for top gear.
A quick look at the gear charts and the perfect set up would be the drive train from a early US spec V8 Discovery. Find a LT77S with its LT230 from a mid '90's Disco I, grab the 3.54 R&P, order a pair of CV joint Series front axles from
HEYSTEE Automotive Components (Netherlands) and you are in business with full time 4WD and Mercedes factory top gearing.
Stock Series ratios wil have you running about 700 RPM too high. Stock with 4.1:1 R&P has you running about 200 RPM too high (but is ball park enough to work if you don't mind the higher low range ratios). An Ashcroft kit in a stock drivetrain is about 300 RPM too low. The engine will be more sluggish below about 75 MPH (!).