bolt both ends of one rail down, push it into place to get it straight. clamp a test indicator on the rail and test the movement against a straight edge.
then bolt the blocks to the moving gantry and get the two ends of the second rail bolted down. push the gantry around while tightening the bolts of the second rail.
rails are usually very stiff in all 5 axis and if your mounting surfaces are not flat then you will be overstressing the bearing blocks without any additional load.