You'd be better off sizing down .450 - 3 1/4" cases and trimming to length.
When you do the initial sizing, set the shoulder length a bit long and try for fit, gradually screwing the die in 1/8th turn at a time to reduce shoulder length until the case rim sits flush with the breech face. You can verify this with a straight edge. This will give you close to zero headspace off the shoulder. I like to then fire form the case. Once done, you are good to go.
For hunting loads, I further reduce the headspace a few thousandth of an inch to ensure easy clambering of the rounds, even if a bit fouled.
Happy shooting!
