Now I thought everyone used vertical slats for the bed, sort of 1/8" x 2" up to 1/8" a 4" supported from below at either end and in the middle. That presents less of a target and the bottom is out of range. Looks really neat and stiffens it up if you put in a bit of a bend. Not saying you are wrong, but it looks a bit odd.

I don't want to rain on your parade, but why are you making it so big and heavy when all you are moving is a plastic torch and a rubber pipe? The routers you see on here need strength because of the cutting forces but plasma shouldn't even touch. Unless you plan to fit a router head, all you have to worry about is the force required to accelerate the gantry. You need at least 0.1G acceleration, 0.3G is good, more is better.

Welded iron looks good in pictures but the tolerances are low because the weld contracts as it cools. If you weld it you are going to have to fit a torch height control and that means hacking the cutter if it doesn't have a THC outlet.