+1 on the 80" or so twist.
A conical load not much longer than a rb can have a great deal of mass so no need to go faster. This twist should probably also stabilize practically any full bore slug on the market over the extent of its useful range.

Brass cases or plastic?

I don't remember the discussion on this board before, but I had the barrel maker cut the groove diameter about the same as the inside dimeter of a hull. Gave me the ability to have a relatively tight fit and take full use of the rifling that was around.005 per side.

Merry Christmas, happy holidays

Tom