Jim,
If you're contect with just one barrel, Lee Jurras built several modernized howdah pistols on the Contender action in the 1980s. I had some contact with Mr Jurras some years ago and he told me that he had stopped making them on a regular basis but that he still had a cuple left at the time. Subsequently, SSK Industries has been building them on the Contender and Encore frames in far more powerful chamberings than were ever made in the old days. The Pedersoli Howdah pistols sold through Cabelas and Dixie Gun Works stateside are nice period pieces, but if you want something really effective, the choice would be to either go to a custom gunmaker building big bore muzzleloaders like Steve Zihn in Wyoming or to have the Contender / Encore versions built but with one less barrel.
Good hunting!
The Ark was made by amateurs. Experts built the Titanic.