Having designed and working with a machine with the C style gantry I would strongly suggest IF possible to move the C so that the Screw is behind it to protect the screw from swarf (I have to clean mine often). While you really do need to increase the rigidity of the gantry supports, that can be done a couple of different methods.

1) go to extrusion for the gantry sides (the solution used on my machine)
2) Increase thickness and add full running ribs to add even more rigidity
3) Increase thickness even more then above and add partial supports to help transfer force to the plate you use for the x axis rails

I would not only go through the build logs here but also go to places like , shopbot, CAMaster, MultiCAM, and a few others and look at those machines and their designs. One of the items about hobby CNC machines is they are often over built. It is not a bad thing but it can add to the expenses without reason. The use needs to be taken into account in the design. Building a machine that can cut steel for doing MDF is over kill and funds used poorly.

Good luck with the design and keep posting, don't mind the gruff from any of the folks here, it is normal and you just take on the information and let the rest go.

Michael