Just IMO.
I agree tall lift spring under has quite a number of disadvantages. The high camber in the leaves make them inflexible and it makes shackle inversion a real issue. You normally have super high spring rate, you still have the loss of ground clearance under the axle from the leafs and it's hard not to make it drive decent (due to shackle angle, leverage on the bushes, castor change ect).
For hardcore offroading there's a number of flaws in the leaf 70 Series suspension design. First off it has quite low suspension roll centres (not centre of gravity) for a 4WD (as do Patrols, interestingly). This gives an uneasy "feel" and more body roll/lean on angles. Due to the length of the leafs and low roll centre design they have a pretty poor shackle/hanger clearance/approach angle. Due to the almost excessive thickness of the leaf packs you lose a lot of clearance under the diff, many 'Cruisers have more diff clearance than spring pad clearance.
On the plus side the leaves are quite long (flex better and have a lower natural frequency) and they're closely spaced (so there's more leverage on them).
Spring over gets rid of the issues related to excessive spring camber, you can gain craploads of clearance under the diff housing, you can have nicer shackle angle and depending how it's done, you can get a higher roll centre and improve hanger clearance.
It's hard to keep a 70 low enough with a spring over, a quick measure on my car shows you'd get 6.5-7" of lift over stock height. Even with sagged stock springs or custom low camber leafs that's still a lot of lift. I wouldn't lift one that high but that's more of a contentious matter of opinion and a can of worms I really don't want to get into. To me it sounds like you want to lift for tyre clearance but a MWB is quite an easy car to modify to clear massive tyres. The rear is easy to tub and the front is easy to cut. I could understand lifting for clearance on something like a 5 door wagon where you have to cut into door sealing area and so on, that's a big load of unpleasant work. You're going to have to move the front diff forwards quite a way to fit those tyres which could complicate things (potential chasssis extension).
You'l likely have to sort out some sort of anti-wrap bar. MWBs have relatively long tailshafts so you'l be okay there as far as vibes go but with a big lift the front tailshaft will not clear the manual gearbox (not sure about auto) cross member). You'l also need a high steer setup (better) or Z bar.
Because the 70 runs quite narrow spring spacing the 60 series diffs (and others) aren't a great fit, the inside edge of the leaf and the pumpkin want to share the same space. You guys live in a great land of custom housings but it's still one of those things that makes it more of a pain in the behind.
Personally, I'd go for a 3 link + panhard in the front using 80 series coil/shock towers (you guys might have better options over there but that's the best option here in Australia) and probably keep the rear spring under keeping everything relatively low. With both done properly (doing the work yourself) I don't think the cost would be massively different and there's a number of advantages. You could run what ever diff housing you want, you wouldn't need to run a high steer, no axle wrap issues, it will drive nicer, it makes moving the front diff forwards simpler, better approach angle, you can tune your roll centre ect.
