Here is a bit that I had archived about how the New One Tonner, a Newcastle company, made 'Crewmans' and 'One tonners' before Holden did.
The IRS was used instead of leaf springs, but the rear chassis was very different.
For the info and enjoyment of those interested.
Each chassis is precisely constructed in a jig that is dimensionally accurate to within 0.5mm. In this pic - looking at the underside of a completed chassis to suit a VT series car - the diagonal fuel tank locating braces and tail light mounts are at the left, the spring pads and suspension front mounts toward the middle, and the prongs that integrate to the floor of the Commodore's original cabin are on the right. Supplemented by the bracing and boxing applied to the rear of the cabin, it creates a vehicle that is far stiffer in beam and torsion than the original sedan.
This view of the side of a Tonner yet to receive its body caps shows the integration of the chassis into the rear of the sedan cabin. The large main chassis members (lower right) disappear under the floor of the cabin, while the tray bed rails terminate in a stout cross-section that is braced from one B-pillar to the other. This picture also gives some of the idea of the space available in the cabin in what were the sedan's rear footwells. The sheet-metal box is a handy cabin storage bin, complete with a shrouded outlet for the retained flow-through ventilation.
The VT-VX Tonners retain the donor Holden independent rear suspension. The chassis replicates the donor sedan's upper spring pads, diff, damper and sub frame mounting locations, allowing the use of standard-style components, (spec'd up, of course, to suit the increased payload). The L-shaped brackets along the top of the rails are to mount the alloy tray that will be fitted to this Tonner. This particular car also has a hand-built exhaust system fitted with high-mount mufflers - usually the standard system is retained, suitably lengthened to match the Tonner's wheelbase stretch.
At the rear of the chassis, the original blow-moulded plastic fuel tank from the donor car is retained. This not only keeps costs down compared to a fabricated steel or alloy tank, but ensures continued compliance with the original evaporative emissions regulations, and eliminates any problems with long-term durability or spare parts. It is also the correct shape for the spare wheel! The box to the mid right is the original charcoal canister also retained for emissions laws. Visible, too, is the self-levelling suspension system pump retained from the top-spec Caprice donor car.
