I think that since you already have Ruger No.1s in DG calibers, you may consider a good old big bore black powder express cartridge for a highwall.
Of course, you could run a reasonable charge of smokeless in it as NfB or light nitro round.
W.J. Jeffery sold highwalls in 50 Express before they had the Farquharson action to use for large cartridges. Replicating one of those rarities has been on my to do list for a looong time.