Handbrake and park (for an auto) normally work pretty good. Although have managed an action stunt sequence of diving into my truck as it disappeared down the steep driveway having assumed handbrake and neutral would suffice (assumption being the mother of all **** ups).
Have parts of an old class 3 - the laminated wood axle has rotted at the mounting point for stub axles. I LOVED the soft sprung ride it provided but at 3.5m long, 300-350mm wide it was very heavy. I have been procrastinating making another axle, thinking of alternatives. Your halved leaf spring looks very do-able and relatively easy to modify the springing rate.
Most likely going to forget my phone number if I keep any more ideas in my head. Great forum!
Did some fabricating yesterday, was able to get a feel for how difficult it will be to keep front stay tight. 
With this length mast will be able to use 4.3m jib, will give a very close comparison between different rigs. The leading edge has me fascinated, looks fast.
