This is what happens when you leave an ox and and Smw3d’s r7 unsupervised…
I present to you the beginning of ROX8!
Be careful, Skynet is here … 
Gantry is 1500mm c beam, and one 1500mm 20x80 v rail. Original ox gantry plates. Feels pretty rigid, probably will be better after bolting together.
I was going to use gt3 belt and pulley, but clearance underneath rail to pulley week be tight. Not sure if you can run double gt3 belts, but I’m gonna try it.
I like cutting out shelves, chairs, and other big things from wood, but there was so much waste with only 2-3 feet of width. Cutting width of this set up should be around 4’
Yup, you can run double belts in the belt & pinion fashion. Better than just a single belt. I am going to the same thing you have done to my OX for the X axis, I figure it will be much stiffer using the cbeam.
I am also changing the Z axis to Cbeam as well. Eventually when I have saved up some more, I will switch out the Y axis to cbeam as well, but that is a ways down the road.
Looking great!