Thanks for the suggestions
Unfortunately, spending 40GBP on two spring sets doesn't fit in my budget.
The Durango shocks are a bit longer than the Schumacher ones, so reversing the adjuster and add a Traxxas spring seat results in a big preload on them. I can reduce that preload by 3.5mm if I cut off the current spring seats entirely. Seems I'll just have to try it out!