I went with side mount rails tucked as much as I could out of the road of plasma dust. The disadvantage is that its harder to align the rails vertically but with a good THC that is not such a big issue.
Here is some inspiration for you
Re your gantry, there is no need to use extrusions. I used a 40mm x 80mm Aluminium Box section. Remember, acceleration and velocity are king in plasma so the lighter you make your gantry the better. I had my ganty ends folded up from 6mm aluminium for this reason with the sides adding some stiffness as the design make sthe ends pretty long.
Here is a bit more recent footage that might give you a better view of the gantry at about 5:20