How heavy is the vehicle? Naturally aspirated 300D ATs have a 3.46 diff from the factory. 2.88 with no turbo could mean a lot of manual downshifting even with aggressive tension on the control pressure cable. You’ll be giving up a lot in acceleration for relaxed cruising in 4th. 722.4 should be fine.

Seems like a lot of work and no mpg improvement to avoid replacing a turbo.

Sixto
98 E320s sedan and wagon
02 C320 wagon