Wheelbase is not an issue as long as you son't mind cutting the cladding. Line up the rear and push the front wheels forward. The issue will be the egine. The straight6 4.0 is quite a bit longer than our 3.5.
As far as going "as big as you want"...if your talking tire size you will still ne limited by the weak axles. The rear axle is way weaker than our 12 bolt and uf I remember eight they have a d35 up front which is no better than our 10 bolt. There are axle shaft upgrades available though that help a little. But your still limited to about 35 inch tires without breakage...unless it will just be a mall crawlee.
Probably be much cheaper to just do a sas with a d44 or d60. Plus then you wouldnt be stuck with 18th century leafspring technology.