I'm not sure of your design from reading your post but I would use 3/4" plywood under the mattress over a frame that would support it well and keep it flat.

Let me rephrase my question - I know I tend to blab on too much and the question gets lost.

I want to use my vintage 2 x12 planks to build a bed platform (boards 6 ft long -running lengthwise)- what suggestions do y all have for how exactly to construct a base or support or frame or some sort of brace underneath these boards so as to hold them straightavoid any possible sagging or unevenness of the boards. Even a one inch warp or sag is too much. They were used in a previous bed frame that had four legs and just a couple 2x4s across and theres a definite sag. Dont want any measurable sag. Thnx.

I'd start by building a sturdy platform frame with 10" wide planks of 3/4" plywood, planks running side-to-side (on edge) every 16 to 18 inches, and 1/2" plywood on the frame for immediate support of the mattress and then attach the vintage planks as decorative facade. Screw heads from attaching vertical 2x2s to be used to join the plywood planks would be covered by the facade.

You might get someone to resaw your vintage planks to half their thickness since they would be merely decorative.

If the wood isn't straight and square, then anything you build with the wood won't be straight and square. Assuming you want to use this wood, the first step would be to joint one face and the adjacent edge at a 90 degree angle to the face and use a planer and table saw to get the opposite face and edge parallel to the first.

If you're looking for a large, dead flat surface the first thing that comes to mind is a torsion box. Maybe you can use your vintage wood for legs?

Use casters that screw in rather than attach with a plate so you can adjust them to the floor.