Oooh - this is not one of those quick answers....
Basically - I agree with cbass - the extrusions are pretty straight and flat - but they are going to conform to whatever structure they are mounted to, if any. The rails are not going to straighten them out. The addition of the rails will stiffen the extrusion structure, but the rails will also conform to the mounting surface. They may not be perfect, but the straightness of the extrusions are probably close enough. If not, they can easily be manipulated to make corrections.
Straightness, flatness and twist are the concerns of the individual rails. For a plural arrangement add parallelism to the considerations. The design of the base structure is key to making a structure that can be adjusted to perfection. All you need is a baseline to compare to for level, straightness, flatness and parallelism, the means to make the adjustments and patience. Considerations in the design can easily provide for the adjustments without sacrificing rigidity.
Scott
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