I was told that the '85's rear end gearing's too high for the 5 speed to work. This is because, unless the gear ratio was changed, at legal speeds you'd be lugging the engine in 5th. Ideally, you want the cruise RPM's to be between 2000 and 3000, preferably between 2500 and 3000. This is where the engine's running at its more efficient.
I know this is a different story with '82-'84 300D's, but my '85 runs at 3000 RPM at 70mph. 2500 would IMHO be perfect, but the engine's quite happy there.
JUMP ON THOSE 5 SPEEDS!!! As mentioned before, they're rarer than hen's teeth...