I'm with gnoahh on this, about not shimming, and wanted to add that most people are overlooking the importance of cheekpiece to receiver fit. If you've ever restocked a 99 you should have figured out that the design is based on recoil being shared equally on the recoil (bottom) lug and the flats of the cheekpieces that butt up against the receiver. In fact, when you're fitting the stock you have to iterate removing material from the top and bottom lug recesses and the cheekpiece flats. The way I do mine to prevent cracks is to get as perfect a fit on the cheekpiece flats as I can, get a good fit on the recoil lug, relieve the top lug by about 1/32", then skim bed the recoil lug and the cheekpiece flats. I'll use a black Sharpie to color the relieved wood to match the color of the top tang; it takes a magnifying glass to actually see that it's been relieved.
