Depending on what you want to do with the rifle can help you decide on what to use.  With a 1 in 12 twist, you can shoot bullets up to 360 gr. Go with a Postell or money nose for long range shooting or one with a blunt nose for hunting. I'm using a 345 g. Creedmoor (Postell) styled bullet in my 1 in 14 twist 38-50 and it shoots very well. Because of the large capacity of your case, I would choose a bullet that sits well into the case that takes up some room. Don't go with a bullet that's got several reduced driving bands that will stick far into the barrel. With that case, you don't need room for more powder.