Regarding what to chamber a custom built Howdah pistol?

I still think a .577 Snider is the way to go.

If original brass is hard to get, I shared a video of someone reloading shotgun shells cut shorter for a .577 Snider rifle. Lead round balls from memory. If this is possible, no problems getting reloading supplies for a Howdah pistol.

Assuming suitable barrels can be sourced, chambers cut etc.

Why not? Stick with the original.