3 door suspension WILL fit a sapph, but you will also have to use 3 door tca's, if using 3 door hubs, all fit straight on.
3 door hub has a LOT less castor than sapphy, can give better grip, but much more twitchy to drive, I prefer the 3 door hubs on mine, there were other changes also to hubs, but castor is the main one.
The main problem area is the REAR arb, but to be honest the arb on a 3 door is crap, so converting to sapphy type would be a lot better anyway.
As you say front legs thicker on 4wd, the eye of the bottom of the rear shocks is also longer by about 1/2" on 4wd, but can easy cut 1/4" off each side as I done this in the past.
Springs are a totally different rate between the 3 door, 2wd sapph and 4wd sapph.
tabetha