I'm (was) running 18 on a bone stock 83 318. you might do the "vac gauge method" & see what that gets you for what it wants for initial. No way would I think 12 would be better than 16 even WO a cam & with it even more reason to bump it up. At least see what the VGM comes up with & try both ways if desired & tune for both (idle speed back to your lowest reasonable hot idle speed along with mixture screws) & see which acts the best then change the slot choice in the plate then springs then adj the can.