Ronan,
The holes that you drill for a 12 valve are the same holes that Dodge factory punches in the 24 valve truck frame, in fact we actually supply you the DC hanger parts. We do it this way as you get to get rid of the 12 valve hanger at the rear of the tail pipe that collects all the mud and muck off of the rear tire and it looks a lot nicer. Makes your 12 valve look like a 24 valve etc and last longer too.
Mark @ DPPI