Hi,
the one super flat and true surface you have is the top surface of the surface plate......so why try to use the sides of the plate?. I can understand not wanting to place anything on the surface and
thereby reduce the work area....but as you have discovered the costs of doing it by using the sides is likely insurmountable.

If it were me I would mount two steel 100 x 50 x 9 or 100 x 100 x 9 down either side and mount the rails on the top of those. If you took the pieces of RHS to a machine shop
it might cost a box of beer to have them put in in the surface grinder to make the top and bottom of the section flat and parallel to each other.

Go to your local scrap yard and ask if they have any cast iron elevator weights. I paid $90 NZD ($65USD) each for three cast iron weights 1000 x 200 x 125.
You'd be amazed at what you can mill out of a lump of cast iron like that. You might for instance be able to make the uprights for the gantry.

Craig