Glad it works and is of some use to someone

Your machine is looking good. what CAD package is that?

With regard the rail orientation then I don't think it really matters for a plasma machine. Mounting them on the side will make the most of their strength and equal out the load capabilities but as you are only moving a light gantry then you shouldn't be getting any where near the max strength of the bearings. There is also the point that the cheaper open bearings can bend and open up even more if mounted on their side.

I've not had anything to do with plasma or even looked into it but it seems to me that if the z axis only has to hold the weight of the torch then there is scope to really reduce the weight of it. Which will help with the deflection and vibration.

I would have thought that you could use sheet metal instead of the plate, get the smallest dia ballscrew (or use belts) and 8mm round rails. Really lean it out.

Is that a double travel arrangement that you have on the z axis, again not a plasma expert but it looks like its just adding extra weight