Joe,

I too will be building a similar style frame to your, with lots of bolting. I think it means not only adjustment, not sure how much, but also allows me to do the welding outside then bring it in to fit together, manoeuvrability I mean. The only difference is I plan on using 90x45 extrusion just where the x rails rest. I think this is an easier method to fix profile rails onto. Not sure if it flatter, but we'll find out when the time comes, but I still do expect to file and sand till its 100% flat. Just don't want problems with the profile rails binding once everything is built.

From what I've read in the past, people use sand for filling in the hollow sections for dampening. Epoxy could be used for levelling out your 100x50mm RHS to provide a flat surface for your rails to rest on. Also could be used on the joints where you bolt the frame together like a thin shims to make sure everything ends up square and parallel. With this method you have to tighten your bolts just enough till the frame is where you want it, let the epoxy dry and harden fully, then tighten the bolts fully.

Cheers all for the heads up on types of cost involved.

Adil