Morning Jayne - Welcome to the Shark Tank. 1) The side webs are not needed and if they are used need to be attached the the edge of the section not the middle. In the middle they are attached to air and will either deflect slightly or can vibrate 2) Same thought on the rail. Its mounted in the middle of the section on air and can move and vibrate. Rails need to be mounted on edges or on thicker material then the RHS so you can thread it and to maximise the local stiffness 3) Brazing is very worthwhile to limit distortion. 4) I strongly suggest not to go down the epoxy levelling path. Again its done with mixed success, more issues then its worth and epoxy is 3GPa stiffness so do you want a rail mounted on rubber?
The side webs chew into your footprint and you will run out of geometry by the time you get to our Z axis design to make the Z axis big and stiff. Its usual to a) start at your max footprint and work inward which usually leads to a disappointing working envelope or b) start at the required envelope and get a bigger footprint then you want. So you have to work around all of those things.
Do you MIG or tig weld? and you have an oxy/acy or propane/oxy torch? If you weld the temptation is to weld everything together. This is unnecessary. 25% or 50% welding is good enough for a machine and you only weld the areas that do not contribute to warping, eg you have used equal sized members and the temptation is to weld where the radiused corner edges are (very big gap) this will pull the structure. Only weld or braze were its flat to flat. Use the next size tube down so all faying surfaces are in contact. Preheat the area with the torch to help limit distortion... People use SHS and RHS as its relatively light and stiff and available. For machine design and build consider open sections. A C channel is same bending stiffness (depth to deph), locally thicker and you can mount things closer to its edge.
Steel SHS and RHS is not flat enough or straight enough to bolt rail to directly. The cars will jam... just been through a similiar thing with YaG and its pressed steel parts.
What you have designed is very stiff and will work fine. The issue is to build it so the rails are level and parallel. If you have a solid welding table that is flat it could be built upside down. Tacked up and do small runs spaced out and slowly work it up. Same process with brazing. If welded once complete you could locally bring welds to red with oxy and local stress relive before releasing from the table. Being in a major city look up heat treaters they can stress relieve the entire frame. But you need to be able to then machine the rail lands flat.
I suggest the walls and table be separate boltable parts so you can adjust them individually. Plus if done in one it will be really heavy. I design things now that can be broken down for transport and lifted easily by one or 2 people. Finding lots of muscle or cranes can be an issue... If you build in parts the individual parts can be machined easier then if monolithic as well....Consider heavy aluminium C sections easy to bolt together relatively straight and light... That's my input for the day... Peter