You will have plenty of trouble trying to get the stock off later.
Do it this way....
Get a piece of pipe or conduit to use as a "tube" for the stock bolt to pass through. Cut it to a length that will fit under the reciol pad. Loose lead shot will rattle around. Use a steel bar for weight. I install the mercury recoil reducers. They add weight and are effective. Put the steel bar or recoil reducer along the bottom (corpal) line of the stock. Stand on the barrel end where it wont be disturbed in such a manner as to have gravity keep the steel bar against the bottom of the stock. Put the tube in in a manner that will allow the stock bolt to be removed later. A piece of masking tape might be needed to keep it in place. The cured foam will hold everything in place. Fill with expanding foam ONLY about 3/4 or 4/5 full. Let it sit UNDISTURBED for 24 hours or longer. Use a sharp knife to trim the foam flush and re-install the recoil pad. If your pad fits inside the stock, you may have to "dig" a little bit of the foam out so it will seat.
HTH, Happy Hunting!
Emri