if doing this again drill the small (3/8) hole on top of the rail and feed the tube up from below. that way the top of the rail is sandwiched between the tube and the new mount so it can't 'shuffle' at all once assembled. basically the same as when fitting a towbar that bolts to a unibody chassis rail, the side of the chassis that the towbar bolts to is sandwiched in place.
neil.
Thanks for the tip :) It's not going anywhere with the bolt torqued down so the spacer compresses between the mount bracket and the washer underneath the rail. I made it nearly a press fit, too.
Wouldn't it be more inclined to move around if the top end of the spacer wasn't secured in the frame rail hole, though?