5 speed gearbox

You would be best to find a complete 75 series ute or troopy to pinch the bits from, lots of changes required with several alternatives.
Some of the very late FJ40 SWB came with 5 speed boxes so that is the first alternative.
Nothing will bolt straight in due to tailshaft lengths being different.
The second way is to get a 5 speed from either a 60 series or 75 series but only some of them will bolt up to your transfer case so a bit of searching required.
The third way is to get the 5 speed and transfer complete but then you will need to change the rear diff housing with handbrake setup as the later models use the rear brakes instead of a transfer case mounted handbrake.
Handbrake cables and setup will also need modifying as will the transmission crossmember, plus the tailshafts. Changing the rear diff housing may also require brake master cylinder and proportioning valve changes as well due to the later drum brakes having different sized wheel cylinders/one instead of two wheel cylinders.
We took the easy way and bought a 75 series troopy at the time.

This may be a better option, Fairey / superwinch overdrive.