You'd need to neck the 12 ga shells down to the 14 ga remainder of the chamber. A chamber cast will show you what's needed. If all else fails Rocky Mountain Cartridge could build the proper shaped shells but they'd be turned shells not drawn.

http://rockymountaincartridge.com/index_htm_files/RMC%20Shot%20Shells%20&%20Kits%20Prices.pdf