Adam,
I'm not quite sure what you have in mind for the chain drive but will see when you get some more photos posted.
Be careful of how much tension you put on the side bearings. They should only really be just touching the rail to keep the gantry straight. If you tighten then up too much then you could smash the bearing or jam the machine when it tries to travel. This could happen because of the chain twin drive system. If both of the chains do not move exactly at the same time and speed then the gantry will slew.
How accurate are you hoping your production parts will be?

I off to have a cry now. just came in from the new workshop that I am trying to get all my kit back into. I have stored all my stuff in the shed and just found my meddings drill head covered in water, ice and loads of rust. A length of flat bar had gone through the side of the roof vent and allowed water to run down it right into the drill head. A simple refit job has now turned into a restoration project.

Regards

Ian