I took the original mount plates off and screwed the pegs into the lower bolthole, the one with the rubber gromet that holde the pipe hanger.
If you want to use the upper bolthole, you should either make special washers with holes drilled for the locating pins, use rubber washers or cut off the pins. It's brobably not a good idea to cut the pins, you might want to put it back to stock someday.

Let's hope there's intelligent life somewhere in space 'cause it's buggar all down here. -- Monte Python