[ricm]

Best method I found it to make a template using some Aluminium angle which you can clamp to the chassis rail. One side of the angle can be clamped to the outside wall of the rail, with the horizontal part of the angle drilled to pre-determine the the distance of the hole from the rail edge. Take lots of care to make the template, making sure you get the lateral location absolutely right, then its real easy to drill the holes in the rail with confidence because you only need to slide the template along the chassis rail to get the right 'front-to-back' location. Much more accurate than a centre punch each time....