The positioning edge needs to be as straight as you want to ever be able to align your axis to, might be expensive to get that machined. The other option is to bolt it down to a flat face and use a reference edge to align it.
The rails are not structural, they expect their mounting surface to do the support work. You might be better off with an extrusion of some sort, or you may need some fairly thick plate to avoid deflection. Time to calculate deflections of different cross sections and compare them with the steel rods already installed.
Those are pretty chunky items, even on a Shapeoko they’re robbing me of machining space by adding the extra 28mm thickness of rail and block (for HGH15 not 20). I’m also losing space to the mounting plate for the linear blocks so that’s another 8mm.
Might be worth comparing with the MGW15 class rails, these are a bit lower in their rated loads but the Nomad isn’t a very big machine.
Big HGH class rails;
Smaller MGN class rails;
Worth doing a proper CAD model to find out how much space you’re going to use for the supports, rails, blocks, ballscrews, ballnuts etc. I suspect your choice will end up being a compromise between ultimate performance and having and X, Y, Z travel left.
Will the exiting controller and motors cope with all the extra rotating and moving mass of ballscrews, plates and linear rails?
Hasn’t Vince already done a heavy duty Z that would be a nice place to start?
There’s no shortage of manufacturer data on what loads ballscrews are rated for;
Not a certain vendor starting with X then…
There are pretty good, flat faced, extrusions available from people like Misumi which many people use as CNC parts. If you want to get really OCD about the flatness and straightness you could do an epoxy pour or shim under the rails to flatten them fully?