So I recently scored a decent McMillan for one of my rifles for a pretty good price.
Problem is, once it arrived I realized that the fore end is just way to wide for my liking.
So the question is........can I take a sander to it, and knock the width of it down? Obviously wearing dust mask/goggles etc. or will that compromise the stiffness of the stock? it's presently 2 1/2 inches wide, and I'd prefer something around 1.75.

No, you cannot sand it down. The fibers that make the stock stiff are located on the outside.

Thanks all, I figured that was possibly the case, but I wanted to 100% confirm.

My understanding is the standard McMillan stock is solid. I cut 3 inches of the front of one and sanded it to get a contour to get my Remington 700 SBr to fit. Forend was solid.

McM stocks are hollow, filled with a sort of foam/bead mixture.  When you order it, you select a "fill" to go in...that fill is made up of beads and whatnot to determine the weight and balance.  Sniper Fill is usually the heaviest fill.

The shell is just fiberglass...paper like material with resin to form into the shape of the stock.  It has some solid areas around the action and where things screw in for strength, but most of the stock is hollow.

Having said that...you "can" cut it down...but you have to have the ability to lay down new sheets of fiberglass or carbon fiber, heat them, bond them, and then refinish the stock.

Some gunsmiths do it but you can almost buy a new one for what it costs...not worth it.

Can you? Yes. Should you? No.