I would recommend you contact Montana Vintage Arms in Belgrade Montana. They are wonderful folks to deal with and can build you what you would need.
There was an article run in isuue #61 of the Black Powder Cartridge News where a fellow had built a Mark III into a .50-2 1/2" for a trip to Africa. He used an oversize Lee Shaver front globe, and a short range "Ukrainian vernier sight" from Buffalo Arms. I presume he refers to the ones available from the fellow from Winnipeg. Red River sights. For the difference in quality and the minimal extra cost, I'd do one of the MVA sporting tang sights. Product #'s 130 - 134, and front globe #111 with interchangable inserts. Good luck, post how it goes. If you like, I can photocopy and mail you the article if you don't have it.