I would ghet the lightes stock available then after the gun is put together ad weight ot the the butt to get the ballance right.
Drill a 1.125" hole in the butt under the recoil pad about 6-8" deep or what ever the stock will allow , drill it as high up close to the comb that you can in case another is needed.
Don't laugh !!!
use a NON Lubricated condom and gradualy fill it with lead shot till the desired balance is reached , then you can either weigh the shot and get a solid lead slug or steel made to fit the hole and match the weight then glue it in place. Also note that you will want to fill the front of the hole with paper so that the weight fit snug against the recoil pad so that it won't shoot loose and bang around in the stock later
light bulb BIGGER NOTE light bulb , As soon as you buy the condoms (only need a couple) notify your wife as to what it is that your doing because leter down the road when she goes digging trhough your tool draws looking for somthing to fish down the garbage disposal and finds an empty condom wrapper in their the $H!T will hit the fan , Turst me