In case you are not aware there is load data for smokeless powder for the 50-140 Sharps on the Hodgdon website. It’s held to low pressure (< 28,000cup) but they still achieve some useful ballistics. The Ruger No.1 action is a very strong action and you could load to higher pressure but your shoulder will likely be the judge of that.
The case capacity is close to 500NE and the Hodgdon site list data for that cartridge too. The 500 NE loads are held to around 40,000 psi and this should be workable for the Ruger.
Cast is probably the best option but Woodleigh make useful 450gr & 440gr jacketed 0.510” pills if you want to use jacketed bullets. Definitely use a magnum primer with a case of this capacity.
I would check the twist rate and throating and work within those limits.
