I have fitted a couple, but always by hand, and on junk shotguns just for practice. Figured I could not go wrong, and would learn something befor I tried my hand on something that counted.
Actualy came up with some reasonabley nice stocks.
My experianc is that the "ready to go stocks" for shotguns are usualy designed for O/U guns, not Side by Side. Those that are parialy inleted for side by sides are typicaly for LC's, Parkers, and other upper end guns. Try finding a partialy inleted stock for a Crescent or some mule eared antique, good luck.
The biggest trick I have encountered is getting everything started square and true. I always started with a stripped action, and inlet the top tang. This allows me to adjust the drop in the stock before making any other cuts. Once you start in on the lower tang your commited because you cut the inlet for the triggers. Start the lower tang, but leave the wrist thick and dont inlet it any more than necessary to hold the action firmly to the stock.
Get the upper and lower tangs fit/ triggers, then strip the side locks and fit the lock plates.
Then reassamble the locks and inlet for the lock work. Finaly adjust the fit of the lower tang to match the lock work. Its easier to cut away wood than it is to put it back.
Finaly fit the trigger guard and cut the stock for lenght of pull and put on the but plate.
Once thats done finish shaping the wrist and fit
Bottom line, its a pain.
As to glass bedding, I have read that Marlin used glass bedding on some of the LC Smiths they produced many years ago. As long as the bedding is not visable on the finished gun, I see no problem with it. It should improve the fit and strengthen the stock if done properly.
Lets face it, the sleak light lines of old side lock doubles create weak stocks. Those stocks can use a little help from modern technology.