Introduction and design help.

[routercnc]

Welcome to the forum

For 400x300 cutting area you would need an extra 200 mm or so on each axis for a moving gantry router, so about 600x500. For a moving table you would need more space to allow the table to move to each extent. So assuming 400 is across the gantry and 300 is the moving direction then footprint would be ~600 x 600. Of course this is a guide so start by drawing the full sweep of the table, add the fixed gantry legs on each side and the Y axis which moves across the gantry and you will start to see what you really need based on fitting all the bearings etc in and spacing them out to give enough support and allow it to be assembled in terms of bolt access. Do you have a CAD package to use ?
Also at 400x300 I’d probably go for moving gantry to keep it a bit more compact but either would work.

[routercnc]

A welded steel frame is fine if you are careful with the weld sequencing, but I would expect it to need some form of levelling still. You can shim the rails level if you have a good straight edge and engineers level, or you could try epoxy levelling to create a flat surface. However having built a few machines and tried different methods I would get them machined flat but sounds like that might not be an option for you.