Frage Steyr 690 (firefighter) - gear ratio?

Hello dear Steyr community - greetings from Poland!

In short, I'm about to buy a Steyr 690, ex firefighter vehicle (with 170 HP and only ~30.000 km of mileage), to build an expedition camper.

The reasons for considering it are as follows:

• it's much cheaper than any other 4x4 truck able to carry ~4 meters long camper on the back
• it was always kept in a fire-station so there is no single spot with rust, no need to do any body/cab/chassis work (apart from soundproofing of course
  
  )
• even being 40-years old truck, it seems to be in mint condition overly, it only drove like ~30.000 km in it's life

My only pain-point after having a test drive was that it's slow. Actually, it's painfully slow...
The maximum speed is around ~80 km/h, the comfortable speed stops at most on 70 km/h...

Having experience in 4x4 builds, I thought about changing the rings and pinions in axles to some quicker ones (the simplest solution) or possibly different transfer case or maybe transmission?
Has anyone done something like this?
Having 90 km/h as a comfortable cruise speed (even with some modifications) would be a deal breaker for me and I'd go and buy this 690 right away 

Best regards,
Piotr.