The steering box does not dictate the turn radius of the Montero, as that is more dictated by the wheelbase and the angle to which you can steer the front end (finite limits based upon the chassis, in this case). The later Gen I LWB steering boxes were a faster ratio than the early to mid SWB models. I put a late 91 steering box in the 87 Raider in 2008 and really enjoyed the increased steering response.

Others have put the Gen II steering box and pitman arm, and idler arm in the Gen I chassis, but I read about other issues resulting. As I have no experience with that, I'd open a conversation with Mongo or others on this board.