don't even bother trying to drive it on the road...the cops will be all over you

narrower axles are better for trails, fullwidth is only really an advantage where you have a lot of room to maneuver
as for the modifications you'll need to ditch the factory suspension up front and run leaf springs, probably need some different springs in the rear as well. and then there's steering, lots of ways to do that. probably be a good idea to use something like a fullsize saginaw box if you want to use fullsize axles though
and just a matter of opinion, if it's not a fullsize truck or a built rock buggy, i think fullwidth axles look like ass
this guy used toyota axles, much better suited to a luv imho