I've got one of the slabsided MkII Rugers, and it shoots like a rifle anyway. If you had a way of putting a stock on one it would greatly improve accuracy. But you'll also be going into the SBR territory as well, meaning tax stamps and such to be legal, IIRC.
Do need a tax stamp on something I make?
I just checked, and it appears that adding a stock to a pistol with a barrel of less than 16 inches is indeed considered an SBR and subject to NFA regs. Some older Lugers and Walthers are exempt, but they were made before WWII
