I understand mech mods have to be inspected/passed by a certified engineer in Oz to register the ride.

In this state, you are supposed to have the ride inspected if you replace more than one major component. Never happens AFAIK.

In my youth, I once helped build a bastard Mini (original) with a 500ish ci Olds Toronado drivetrain. And a 6-71 Jimmy with 2 4bbls sticking out the roof, which got crushed when a particularly vigorous wheelie flipped it over backwards. 1700lbs driver up and wet. About 700hp. We may have invented wheelie bars, removable for "street legal". No nanny state would ever approve that monument to excess. As my buddy and fellow criminal in the above crime, Erasmus Dooty said, "If mo' is good, hell of a lot mo' is mo' bettah". Except you had to park it if it rained...

We once put some mud tars on the back, and won a mud bogging event, by going so fast we just skated acrtoss the top, trailing two beautiful rooster tails of mud.

And in answer to the original question, Yep, there used to be some, before the trucks got too old, but the only real source of aftermarket support can be found in your bathroom, in the image right above the sink. Reference library is right here, but it has a crappy Dewey Decimal system...