Hi - I like your gantry uprights. They make efficient use of their space and will be very stiff with forces in the left-to-right direction. But you might want to add a gusset that runs from the bottom of the gantry down and out to front linear bearing. That would stiffen it in the front to back direction. Also have a plan to true them up after they are welded. Welding warps. I built welded uprights for mine and compensated for this with conformal shimming with epoxy. You'll also want to build in some accommodation for adjusting squareness. I'm not familiar with the MASSO controller so I have no opinion. But I can whole heartedly recommend UCCNC software with a UC300ETH controller and a UB1 breakout board. Good Luck with your build.