My experiences are with metal, not wood, so keep that in mind as you read this!

My first course of action would be to fill the crack with thin glue and as mentioned, clamp the stock to close the crack.

To stop a crack in metal, you drill a hole at the end of the crack. I suspect this would work with wood as well, but for obvious reasons, you wouldn't want to do it from the front of the stock. Measure the thickness at the end of the crack, then transfer that measurement to a 1/8 inch drill bit, subtracting an 1/8 of an inch for insurance that you don't drill through the stock, and drill the hole from the back using a manual hand drill like hobbyist use. I would then inject epoxy into the hole, using a toothpick to make to remove any air and pack in the glue.

If you want extra strength, you could "vee" out the area along both sides of the crack on the inside of the stock using a Dremel with a cutting bit, and fill it with a good epoxy. That is how cracks in metal are fixed, only welding is used of course instead of glue. Although it can't be seen from the outside, you can finish the epoxy back to the original surface height, which may be necessary to obtain proper fitting of the stock.

Brownell's has decades of stock repair experience and could recommend which glues would work best for you. Just make sure you speak to one of their smiths and not one of their phone sales people! As a side note, I've had some amazing fixes on wood, metal and plastic using JB Weld. The stuff is very strong and can be worked back using files or emery boards.

Take your time and I'm sure it will turn out fine.

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So many S&W's, so few funds!!