For cutting aluminum if gantry is from alu profile, then make your gantry as Dean makes his machines, L shaped. To simplify things for you, check data for 2x 100x100x4mm or 200x100x4mm on the span of your intended gantry and that should be your minimum, aluminum or not. I am talking about the moment of inertia of the beam.

I have a complete drawing of my machine in Sketchup which has 1300x2600x200 mm working area. Later i revised it again piece by piece as i sold plans to some interested people and reflected the changes and what i have learned from the build and even further simplified some stuff for easier experience of putting it all together. Also corrected some details for the cheaper Chinese servos to fit there. I believe $ per strength my machine is an incredible value, where 700kg of metal do what normally 3 tons of metal do. I was contemplating also to redesign all instead of Sketchup to make it in NX or other CAD. So its easier to visualize and make a manual with instructions how to put it together.

In other words i dont mind giving the CAD for free but in order to update it with variants like L style gantry, etc. its an enourmous work. So i have to be stimulated to develop all that further, as its absolute stupidity for people again and again to try to reinvent the wheel for months when i have spent half an year designing fast and strong CNC which i know every little detail about it. The fact that i could machine mild steel 1mm DOC with Z axis fully extended and have perfect finish should speak enough about the quality of all. I am not selling you sth here, but if you want to save time and effort and help further with the project, PM me and we can arrange sth.