Well that is a good question.
First I started with the y-z plate and mounted that to the rail trucks then i bolted the rails to the tube, this auto-magically aligned the rails to each other based on the mount plate. I then slid the plate down and tack welded the support bars as i went. As for the bars to the beam i started with the highest grade steel i could find, i then torqued the bolts to 80 foot lbs. The rails are 45mm hardened steel,very strong and ridgid. I was hoping the rails would make the beam conform to them, i think it worked. It travels very smooth with no apparent binding. If the rails have some hi or low spots because of beam fluctuations i should be able to shim it out. I am only trying to hold a couple of hundredths tolerance, there will be much more slop in the later fiberglass process on the parts.
Chad