ok, now i understand.. i orivinally drew a backside angle design... you can see the pics here..
www.jpcustomcrafts.com/shopmachines.htm but i wasnt going to use an upper and lower.. only the lower... but i have modified this design so that i will be riding the edge rather than the backside of the angle... this way i can adjust the angle for straighness... as you will see in the drawings, i was going to use a pinch roller to ride the underside of the top rail to hold the gantry down against the angle...
im not seeing any adjustment either in that design, but my suggestion would be to do like i have seen done, and mount the angle in some fation so that it is adjustable, and not so much the bearings.. altho you could do this as well.. i forget who it was that i saw doing it, but someone build a metal router, and used round rod, to mount the roundrod they put the ends of the rod in these blocks that had 4 bolts comming out of it at 90deg apart.. this way you could adjust the angle up and down, and left to right to get it all square... maby if you welded some short peices of box to the end of your angle, then you could put the box inside this adjuster plate, and that would keep it from turning, and allow some adjustability...