The good news:
The Ruger #1 in .450/400 NE is an accurate rifle.
The long neck handles cast bullets well, and obturates even at low pressures.
The bad news:
Brass is expensive at $2 apiece ($1.50 if you can find it on sale).
Here are my reduced loads. They worked for me, but you must decide if the risk inherent in handloading - and creating reduced loads - is acceptable to you. Use them at your own risk.
Rifle: Ruger #1
Brass: Hornady
Primer: S&B LRM
Accurate Molds 41-370B bullet. About 364 grains with my scrap alloy. Seating depth 0.596"
SR 4759 26.5 grains, 1335 fps
SR 4756 15.5 grains, 1075 fps.
Mountain Mold 445 grain bevel base (about 442 grains with my scrap alloy. Seated into case, depth 0.750")
Steel 20.5 grains, 1200 fps
SR 4759 25.0 grains, 1185 fps
All of these will shoot as accurately as you can hold. I size them to 0.411" and have had no leading.