Presumably the rails are drilled at the standard 60mm spacing... guess that could get tedious over 20 feet/6m - approx 100 per rail... but only needs to be done once. Guess you might get away with every other hole (120 mm) but doing them all would add a little to rigidity I would think...

Did you find the Hiwin rail in 6 m lengths BTW or are you going to have to butt two lengths up together? Longest I've seen is 4 m at e.g. WMH Transmissions Ltd - Product List - HGR 20 GUIDE RAIL<BR><BR>PITCH 60<BR><BR>NORMAL GRADE