Welcome to the forum. 336 conversion to 445 supermag----interesting project. Be CAREFUL ! I shoot big bore handguns and I have a Dan Wesson 445 supermag with a 10 inch barrel. I made the mistake of not buying barrels and shrouds when they were available. When you get into the high pressure big bore rounds like 454 Casull, 475 Linebaigh, 445 Supermag, 480 Ruger, 500 S&W you have to make sure you use bullets with a jacket made to withstand the high pressure. You can have a dangerous situation if you use select a bullet that is not made for the high pressure. The bullet will smash up at the back of the barrel when it hits the forcing cone and act like a barrel obstruction which can raise the pressure to a dangerous level. Make sure the 336 receiver can take the pressure.