Dion will turn up here soon enough. I would guess that it really is only the linkages and gearstick you need to muck around with, and the stick should come with any five speed box you buy. My manual (from the states) doesn;t say there is a different gearbox for column shift versus floor shift, nor for four vs five speed (same box, different internals obviously), so I reckon clutch slave and tailshaft will be alright.
Measure up the length of your gearbox from bell housing to end of the transfer case and I'll compare it to mine if you like - I've got a five speed floor shift.
I've got both a D21 Navara and Terrano, one manual one auto, and in both cases, the hole in the floorpan for the gear stick/t-bar is identical, with different plates bolting in as required, so I reckon you'll be right there too.
Might be able to help out with steering column bits too if you want. As long as its not a rush job
A lot of work to get an overdrive gear. You could just stick to 90 on the highway instead.
What's the condition of the box?