Congratulations!
The 450/400 3 1/4" Nitro Express Magnum is a great cartridge.
I have killed deer, pigs, caribou, and black bear with mine.
Loads that regulate well in my double are:
With 400 grain Woodleigh Softs, 81 gr of IMR 4831, 83gr of IMR 4831, 71.5gr of RL 15.
I have had very good results on deer and pigs with 300gr Hawk bullets with the .025 jkt with 81gr of IMR 4831, vel. 2332fps, and 71.5 gr of RL 15.
Also for a lighter load ,velocity @ 1700 to 1850 fps, I have not chronoed it yet try the 300 grain hawk with 46 grains of IMR 4198.
You MUST USE FILLER with RL 15 and IMR 4198. I use foam that I cut out with a 50 BMG case. The filler must be compressed. Keep us posted on how your rifle shoots.