fisherman983,
According to their website, McMillan currently doesn't offer the integral adjustable cheek piece for this stock style.
Attempting to cut the cheek piece off of a finished stock can result in de-lamination of the fiberglass outer shell from whatever the fill that was used.
McMillan uses 4 molds to make an integral adjustable cheek piece stock; 2 for the basic stock but minus the cheek piece and 2 for the section of the cheek piece. The area where the cheek piece is supposed to go is a solid piece of the shell so that it won't de-laminate.
One solution to your question would be use a Karsten adjustable cheek piece. This can usually be installed DIY if you have some skill with a drill.
Regards.