Bill,
This must be in relation to your re-barreling thread...
Assuming you bought the rifle used with the stock and don't know what you have?
What contour barrel is on the rifle currently?
Usually, one gets a stock with the barrel contour to match the barrel, unless they know they're going to put a larger dia. bull or varmint barrel on it later.
How much taper is there in the barrel channel? A full "bull" barrel will have no change in contour down the barrel channel. Full width from the barrel nut to the end of the stock. Varmint contours taper from about 1" to .8", so there might be a slight narrowing.
If your stock is set up for standard contour, I don't think it will be feasible to modify it because it's fiberglass. With a wood stock, you can take a piece of PVC pipe and sandpaper and enlarge the channel. With the fiberglass you have a very thin glass "skin" over a core, and enlarging would require professional work/re-glassing of the stock.
If that's the case, best to sell it and buy a new one inletted for the barrel contour you need. All quality stocks get pretty good $$ at resale, so it's not a big loss...