If 1st gear is low enough to support this, it might be possible to modify the diff for a higher final drive ratio across the board.
Might be cheaper than a tranny.
Of course, your speedometer would need to be recalibrated as well.
Pretty flat in most of Oz, right? Higher 1st (and reverse) gear may not be a problem. Certainly would save some petrol.
1986 300E 5-Speed 240k mi.