Not that easy a job from what I can remember when working on an LJ a mate had many moons ago.
He had a mega-hot 202 LJ and I was driving a Mazda Capella...I lusted after an LJ two-door but even then they were hard to find, but I could have got a four-cylinder LJ cheap from another guy we knew. But this created a problem...the problem is that the four cylinder LJ uses completely different front panels than the six cylinder...the grille goes straight across at the front instead of having the "pointed nose" of the six cylinder body, and I am pretty sure even the front guards and bonnet are shorter. From the firewall back, the cars are identical...from there forward, they are completely different.
This means therefore that it isn't a simple matter of dumping the four and slotting in a six...it is a major body modification job to get it in there.
HOWEVER, don't let that put you off...I am reasonably sure that the front crossmember of the six cylinder with the good disc brakes and steering will fit straight in (someone stop me if I'm wrong) and then this leads to finding a motor as long or a bit longer than the original four cylinder...feel like hunting around a wreckers for a good V6 Commodore motor and manual box? (Reasonably certain that the auto will need a lot of floorpan mods to fit...).
Now that would be something different...maybe a bit costly, but different and probably easier than trying to find a good set of front 6 cylinder panels, even if they could be made fit the shorter four cylinder body.
Coincidentally, there is a very good four cylinder LJ two door just down the road from my house which appears to have been painted and partly assembled and is for sale. It comes with heaps of parts and is, I think, about $800...now if that was a six cylinder LJ 2-door in the same condition, that price tag would not be $800 but more like $2800...at least.
Good luck.
