Why not go on-line, measure the vent rib in the picture, and calculate where the spacing falls. I got between 18-19 inches, which would be perfect, but I would like someone to check my math.
The Comfortech stock is highly desirable. I have heard of people cutting the non-Comfortech, but given how the pads slip on the Comfortech stock, I don't see how you can shorten it and maintain the same pad set-up.