Yogi:
With those ballistics, the .50-90 Sharps should be a marvellous North American conversion, go for it by all means, but I respectfully suggest that 20-inch barrels will feel too short for a hunting DR. Go with 24-inches if you can, this is still very short as far as double rifles go, but will be much nicer to shoot with. You might get away with 22-inches if you spend most of your time in the thick stuff, but 20-inch doubles are for the 'Cowboy Action' shooters IMHO!

Of course, it's your dollar. Let us know which way you jump. Pictures are always good!