Hi mate, welcome to the board.
Funny, there seems to have been a rash of SPOA questions lately. All of a sudden, it's like the clock's been wound back 10 years.
Firstly, Kenn is right on the money here - SPOA is a very involved conversion, and an engineer will be very specific about what you can and can't do. It's going to be hard to keep overall height within an approved height, and steering system modifications are generally very, very carefully scrutinised. It's quite possible that none of the important or expensive parts of that US made kit would be permitted by the engineer, or may require expensive testing before the individual parts are approved.
However, an engineer won't ask you the most important question of all - why do you want to do this? Their job is to put the legal stamp on your project, not to tell you it's a great idea. In fact, they may have no idea at all why you want to do your chosen conversion and aren't likely to care - they will just ensure it meets minimum safety standards.
Another note that's worth mentioning.
Whilst some Victorian engineers have approved SWB sierras with 31" tyres, it's increasingly difficult as it's a little bit of a rule bend. In truth, if your car has a 15" tyre placard (likely to be for H78 15's) the biggest tyre the engineer can approve is a 28" tyre, and that's not really worth building a SPOA car for. If you do all the engineering work and then run a 31" tyre, you car isn't legal and isn't approved, should you be subject to an inspection or involved in an accident.
If your car has lots of custom fabrication on it, then there might be a good reason to certify the car and then run a bigger tyre "for offroad use only," and there's no problem doing that, it's important to recognise an engineer can't legally comply any tyre you are likely to want to run - especially with the 5" of lift a SPOA provides.
Also, I assume you're aware that your car is currently narrow track. You'll really be wanting to swap to wide track axles early on in the piece. They aren't ideal for stability and are a strength upgrade dead end (and I'm not sure you can buy a hi steer kit for NT axles for good reason
)- there's also no heavy duty axles available. You can keep it narrow and it will be plenty capable, but you'll really want to keep it low. So, to start with, I know you'll be super keen to "rip into" your project, but take a step back, do a lot of research, and really plan your build out. You'll notice on here there's very few SPOA on here. Lots of SPOA cars were built around a decade ago, some were quite good, some were terrible, but all of them generally took lots of time to get sorted, (if they were ever properly sorted), to the extent that plenty of people moved on to coils/links or back to SPUA rather than continue down to try and sort out a SPOA car.
The trend in car building over the last ten years ago has been that cars are being built lower, and tyres are getting bigger. 31's and a SPOA (sometimes with lift springs and/or a body lift) were really common back in the day, but the spooky road handling, short spring and uni life, and pants-dampening off-camber performance meant the novelty wore off for lots of people, and probably more so in Vic where our terrain is so off-camber and the mud means we struggle to pick clean lines.
The thing is, it's easy to run a 31" tyre with just a 2" body lift, and the low centre of gravity that results makes for a trustworthy car that's easy to build and critically, easy to drive on road as it doesn't attract attention.
In my opinion, tyre size (for diff clearance and traction) and a low centre of gravity is where it's at for our terrain. It's really hard to build a car that's SPUA with the stability of SPOA, so some of the apparent advantages of SPOA don't look so great if you're packing it because the car feels like it's going to roll. A really experienced driver can learn to drive through the spooky and get good performance, but that's kind of perverse if the lift isn't required or adding to capability.
I know you asked a simple question and this is far from a simple reply - feel free to pay no attention - but it sounds like you're pretty new to building sierras and it would be a shame if you got some way down the road of a build up and weren't happy with the outcome. Much better to learn from others experience than your own.
So, how much tyre do you plan to run, and how much experience do you have with suzuki's off road? If it's not much, you might be surprised at how capable a really simple build is without having to totally rework the steering and suspension. For my money, throw a locker in the back, a 2" BL, some 31's, some transfer gears and drive it, and start building up experience about what you'd like the car to be able to do.
I've been playing with suzuki's for almost 20 years and building my own car for 6 years, and I've just got it to where I want it, and that's as much about me building my skills and experience (and therefore changing how I want the car to work) as it is about the car itself.
Steve.
Oh and check out http://www.vic.suzuki4wd.com, We don't bite
