Earlier on I noticed mention of torsional stiffness of the bed.
The stiffest thing (IMHO) is a ROUND TUBE.
Maybe you can build the whole bed based on a big chunk of pipe.
Now slide all of your stiffening webs on, align them and weld to the tube.
Now weld the flat faces to webs. Very easy keep it all lined up after tacking the webs.
Any comments?

If you intend using boring bars (and a turret) , consider that the Z travel has to be twice as logn as the hole you want to bore, plus the width of the boring bar holder. Look at standard size boring bars, or every time you want to use one, it will be too long or you will only be able to do very short jobs.
A standard 300mm bar really needs a 650mm bed to be useful.
If you do not use a turret, or do not rotate it then longer bars fit a bit easier.