I knew a guy who set up coils front and rear on a 2nd gen 4Runner back in the 90s and he put it on it's side on the first test drive... on the street! He ended up goin to leafs in the rear and it helped alot.
I think it could be totally viable to use maybe some FJ80 stuff in the front but you will have to maybe use leafs in the rear and possibly a sway bar or you will have alot of body roll. No at all saying it can't de done but some extra engineering is necessary vs. leaf springs. I'm sure staying low oon the ride height would solve most of the problem.