Set to the length you want then fit them-they cannot move ie shorten or lengthen once fitted (as they have opposite threads)
I had the same worries/ conundrum when I bought mine- but they are fine.
I set up with Stuart this morning- it is pretty dry up there- ground is rock hard. I don't think there is anything to punishing vehicle wise. Got the see-saw included in one section.
I bought quite a bit the last time I was there- they just weigh it and pretty much charge the scrap value- I want to go back and get some box to make tree sliders.
I left yesterday afternoon as I knew it would be bedlam gettng out today- even with MT's it took a bit of effort. I have no idea how the Nissan Micra half way down the hill got out- it would have fallen into some of the ruts :p