You laugh at string levels, but watch what the guy in the video below does to level a welding table.

https://www.youtube.com/watch?v=kjaSy_SmYsQ

If all the pads (upright and rail) are at the same level, it seems to me it simplifies the machining. For good enough, you would work off a flat surface with machinist level to grind your way to perfection. Blanchard grinder ("Our largest 100 HP 60 inch chuck Blanchard grinding machine will cut the stock quickly and it allows us to grind parts up to 72” diagonally.") or even surface grinding ("We offer tight-tolerance surface grinders with sizes up to 32″ x 96″ and 16″ x 168″) as options at large outfits. They can also do stress relief if you think it needs it and/or don't want to setup a bake oven.

But, yeah. This is one area that I haven't seen discussed a lot other than people using large mills where they work to complete the flatwork. I have a crude surface grinder fixture idea bouncing around in my head.