In my case bolting the rails directly to the 80x80 box section seems to have worked.
The is a slight amount of binding at one end but I think I can attribute that to the way I've fixed the box section to the gantry. The box section is currently only bolted at the vertical sides which has squashed it very slightly causing the top and bottom to bow, pushing the rails apart. I think If I put some more bolts in it'll be fine.