Thanks for the input!
As a matter of fact, the springs have been changed to a series 60 spring-stack. These are stiffer (since the 60 series is a MUCH heavier beast than the 40), and longer too. Also, we have HAD to outboard them somewhat, because the series 60 diffs NECESSITATE that the shackle / mutthi be mounted wider on the frame. Not by a great deal, admittedly...just 1.5 inches on each side.
There is NO doubt, however, that not just has the ride height increased, the centre of gravity of the machine has also grown higher. This poses a delicious mental dilemma, which I'll attempt to explain, by showing various (somewhat contradictory) dimensions of the Phantom.
a. The SOA has added about 6 inches to the height, whereas overall ride height has gone up by about 15 inches (due to curvier leaf springs, and oversized tyres).
b. Spring "stance" has been widened by 3 inches.
c. Weight will go up by about 400 kg, unladen (additional coachwork, frame extension, roll cage, interior...combined with some weight saving on the engine, probably)
d. The diffs are 3 inches wider. The overall increase in width, though is 20 inches (because of tyre offset). Whilst we do not EXPECT any rubbing by the wheels, 2 inch spacers on each side (if absolutely inevitable), will accentuate the width increase to an even 2 feet.
e. Wheelbase has gone up from 90 inches to 114 inches (increase of 2 feet). 6 inches have been added to the tail, thereby increasing the length by a total of 30 inches.
f. Original approach angle of the FJ is in the 55 degree range. When finished, the Phantom will have an approach (and departure) angle in the 75 degree range.
You'll agree, that this is mouth-watering stuff. Also, it makes for a lot of "ifs and buts". WILL the Phantom be prone to a great deal of sway? Possibly. Perhaps not. It's touch and go. The jury is still out on that one.
The lack of any empirical evidence to suggest a straightforward "yes" or "no" answer is why we're putting "controversial" conversions / add-ons, on hold, for now.
My "rebuild philosophy" in this regard has been fairly simple. What is absolutely essential is modified / changed / outboarded / removed (and pretty much EVERY bloody thing HAS been modified!). However, all those things where even a marginal doubt exists as to whether or not they will be efficient / efficacious, have been left untouched, until the Test & Adjustment phase of the rebuild.
This phase is going to last for 3-4 weeks, and by God am I going to be brutal with this rig, during that phase! In my mind, I have a check-list of sorts, regarding what aspects I'd like to thresh out, thoroughly. The stabiliser / sway / anti-roll / panhard bar options are very much a part of that. Offhand, I've also heard of some ride-adjustable shock absorbers from someone (I don't have any personal research on the subject)...which could prove useful in the offroad / onroad ride quality aspect. I've also put that off, until later.
Your post serves as a wonderful mental springboard, though. As does every single post which is added on here. It makes me take stock of things, and even if they HAVE been considered, a fresh re-evaluation in the light of comments like yours, infuse a deeper understanding of the task in hand.