I have no problems getting brass and the 450 brass is tougher 48gr of 5744 and a 300gr bullet rocks.
According to my loading manuals, you are way over max with that 300 grain bullet. Please be careful, if you want to shoot 45-70 levels, then get a 45-70. Tougher brass, means less case capacity. The same problem happens with my 444 Marlin squirrel rifle, the brass is thicker and smaller in diameter, so it can not make the same velocity with similar bullet weight. It does not mean the 450 or the 444 are no good, it just means they just will not do what a 45-70 can do. Either one will take any game animal on the North American continent and do it with authority, as long as you keep ranges reasonable. How dead is dead? Again, please be careful. If you really need a higher level of power, then get a different gun that can do it safely.
Good Luck and Good Shooting