According to the specs, a 5-speed was an option on the 4-cyl wagon, but all
the wagons I've seen (pretty rare too) have the automatic. My interest is
that the wagon 5-speed has a lower final drive ratio compared to the sedan.
Both have the same gear ratios otherwise.

Since it looks like finding a wagon 5-speed at a wrecker is pretty unlikely,
getting new parts and fitting them to a sedan 5-speed is the solution.

Assuming the trans is on the bench, what's involved in changing a final
drive gear. Obviously the case has to be split. Is the smaller pinion gear a
removable item or is it part of an intermediate shaft? I don't yet have a
workshop manual to look into this in detail.

Why? I've come across a cheap 97 Odyssey (first generation) with a very nice
body, decent engine, but a fried transmission. I'd rather convert the van to
manual (yes, I've done this sort of thing before) than fix the auto trans,
and the sedan 5-speed final drive would be to tall for decent performance.
The wagon trans final drive would be preferable.

SJD