The only fitting that needs done is the forend barrel channel will need a little fitting to the barrel as they leave them slightly undersized for the factory barrels, if you use a heavy or bull barrel there will be a little more wood to remove, I use a piece of sand paper wraped around a wooden dowel, they ( Gunstocks Inc. ) fit a frame to each stock before it is shipped so there will be no fitting required there, they sand the stocks to around 80 grit and you will need to finish sand to 220 or whatever you want.
I have purchase several stocks from Gunstocks Inc. and am very pleased with there product, they are nice people to deal with and very helpfull.
The last stock I got from them was their Stinger thumbhole and it is the best thumbhole stock on the market as far as I'm concerned.