Hello Rick,

I do not have experience with the 450 Rigby, but I load the 460 Weatherby down to the 450 Rigby level and I can recommend this velocity/energy range.

My load is the 500grs Hornady for about 2360 fps, and I had the luck to place 4 shots into 40mm at 100 meters (rifle rested on bench).
Recoil is available, but acceptable.

The 300grs Flat HP from Sierra and Hornady and the 405grs flat point SP are not recommended for hunting purposes with velocities above the 45/70 level, but for plinking the are very useful.
The 350grs Speer SP is describend in the Speer manual as suited for the .458 Win, so that would be an alternative for light game.

The Brno as a basis for the conversion is very good, but I would replace the factory safety because it works the wrong way.

best regards
Rolf