Thanks, Crhis for the diagram. That's exactly what I needed. I'd love to build the whole thing out of steel just like yours but I have neither the skill or the proper equipment to build and move it around.
I observed that the machine rigidity has less to do with the build material (steel, AL extrusion, wood) and more with how each part is interconnected. Welded steel has the most solid connection but welding stresses/distorts the metal. Extrusion with T-nuts alone isn't very good if it relies only on friction between two surfaces. Epoxy with t-nuts will get AL extrusion close to welded steel rigidity. If epoxy isn't possible, perhaps a layer of fine grit adhesive-backing sandpaper between the two mating surfaces would improve its connection rigidity. Weight is also important in damping any vibration caused by gantry inertia. However, weight is probably better added at the base, not the gantry. I plan to add some concrete blocks to the base when everything is done.
GaryLa - I use about 55-60 linear feet of 16ft wide LVL. Those tends to run about $9-12/ft retail. I got the wood from craiglist for free but the wood had some damages and had to be dressed. You'll find LVL on craiglist from time to time for very little money, especially the shorter sections.
Here is a picture of me cutting the steel stretchers - all 8 of them at the same time. The saw did not like it but I need to make sure they are all at the same length.