I have been thinking about an affordable reasonably accurate track system as well. One of the solutions I am considering is to take cold formed square stock and place the sections end to end rotated 45 degrees with machined supports made with a "V" top and a hole through the center to allow a bolt to come up through the flange on channel or I-Beam supports and thread into holes drilled and tapped approximately 50% of the diagonal of the square stock. This would allow one to place shim stock under the supports to compensate for the variances in the channel or I-beam supports. Additional support with a tapped hole would be placed under each junction of the square stock to insure alignment under load. Supports would need to be spaced such that the calculated deflection of the square stock with worst case load from one carrier would be minimal. If you plan on a rack and pinion drive with a spring or pnuematic preload mounted above this track on the opposite flange from the track you should be able to go with one track per side. I would think this size gantry would demand a rack and pinion and drive on both sides.
For this large of a system it is definitely worth looking at cable drive for the x axis for the sake of economy. If this is done it will require the mounting of a second track inverted above the first one in place of the rack and pinions. You will want to design the v roller system for the upper track to have a spring or pnuematic preload system as well. The cables on each side can be driven from a common shaft and one motor. As long as the cables are pretensioned properly the positioning accuracy can be impressive after a short initial period of time where there will be a small amount of stretch until the cable "sets". You will need to machine drive pulleys for the cable that allow at least 5 complete wrapps of the cable around them winch fashion to make sure there is no slippage.
If you are determined enough this system should be able to be shimmed in to an accuracy of a few thou at any given temp despite the large size. If you find something that looks better for the money let me know.