pc:
Assuming the cartridge is .500 x 3 1/4 Express (straight case), then you should start with 72gr Hodgdon's Varget and a 440gr thin-jacketed or hard-cast bullet. Wad-up to fill the air-space, either foam, saddle-felt, or a big ball of dacron.

That load is 'factory-equivalent' for the 3-inch case, so be prepared to add a couple more grains for the extra case capacity. Hopefully the extra 20 grains of your 460-grain bullets will not make a significant difference, otherwise they will want to shoot slightly apart. Pressure should be well below 10 tons.

Use a chronograph, and don't exceed 1850 fps without really sitting down and thinking about what you are doing!

...and chuck those bloody 550-grainers away! The rifle has already been proofed!