Moving bed - bearing spacing?

[Snapper]

So... after a bit of thought I'm going to revisit this idea, come up with a design, look at the advantages it gives me then put a cost on it and see how I feel about it.

I'm going to work around all the stuff I already have, as I have a full machine with 20mm profile rails, 1605 ballscrews, and try and come up with a better frame and gantry assembly to add rigidity.

I mapped out rebuilding the machine in its current format, just with using far beefier extrusions than the floppy thin things it's currently made of (the gantry beam is 20mm thick t-slot extrusion, 15mm gantry arms). I realised I'm going to lose work area anyway and like the idea of having a moving bed and the freedom to add all sorts of bracing to the gantry so I'm going to look at it properly.

I already have a solid 725 x 480 x 20mm bed made from aluminium tooling plate. The rails I have for it are 700mm long 20mm hiwin rails, and 4 HSR20A blocks. I don't know whether to bolt the rails directly to the bed or build a frame under the bed with the rails bolted under the bed frame. Bearing spacing is something I have no idea about. Should they sit at the edges in the width or maybe a third of the way in? Then in the length have a 150/200mm gap between the carriages which are 74mm long, giving a travel of 350/400mm. This is obviously where moving the gantry or the bed will be decided.

The other thing I have to decide before thinking more deeply about it and coming up with a plan is construction material. The tools I have at my disposal are

6x4 bandsaw
Angle grinder
Drill press
Nuts and bolts
Current CNC router

I can get a mate to do any welding but he reckons it will be difficult to get everything square and straight and will likely twist a bit during welding. How do you folks who have built steel frames manage to get everything properly square, and how do you manage to get a flat and coplanar surface to mount the rails on? General lack of experience working with steel is also going to make things more challenging. I can foresee costs escalating sharply and rapidly as I end up having to send stuff out for professional machining/correction.

Aluminium profile, whilst weaker and the base material more expensive, is easy, I have plenty of experience using it, I have my current machine that I can use to mill flat keys in for the linear rails. If I go with the fixed gantry I have the freedoms to belt and brace it to my hearts content. The cost would be purely material cost and any processing I can do by myself. I would probably go with heavy gauge 90x90 Bosch or 80x80 ITM profile.

The other option is just rebuild the current design using 80x80 and 25mm end plates/gantry arms etc which I priced up coming in at under a very reasonable £300 including fasteners and any extra tooling I would need (drill jigs).

The main goal is to get a better surface finish from cutting aluminium. Faster feeds and bigger cut engagement would be more of a (much welcome) added bonus.