That would be another cool project!
I don't know if the Finnish L-35 was equipped with a shoulder stock, but the Swedish M/40 was not. There were some prototypes made, but I have never seen any proof that they were ever produced. Kind of like the Radom VIS: Slotted for a stock, but there's no stocks available.
I suspect that you want to make everything yourself, but there is actually a reproduction available. I e-mailed them about this a couple of years ago, and they said that it's based on the original prototype. Can't remember if they actually had drawings, or if they were going by old pictures.
You'll find some high resolution pictures here, they should give you a pretty good idea of how it's made:
http://www.stockholmsvapenfabrik.se/...roducts_id=400
Edit: If I'm not entirely mistaken, the M/40 doesn't have the accelerator. The L-35 does, but it's not really a pistol you want to cut up.