Hi CNCexplorer,
Excellent job you have done and it looks like you are well on your way to making a very useful machine. As I am sure you have already found "everything is a compromise" and usually with tight financial restraints.
The gantry slide system you have used is extremely difficult to set-up so that it is fully true and parallel but others, before you, have succeeded with this design and achieved excellent results. Should It prove troublesome and assuming finances will allow, you could perhaps consider fully supported linear rails (similar to attached pic) as these may, I think, be far easier to set-up.
Tweakie.