There are a lot of 1840-1860 trade guns to copy or style a gun after if that is your time period. The "Leman" is just one of many that happened by the luck of the draw to be popularized in the 70's by a fine copy made by Green RIVER (not mountain) Rifle Works. I am not dissing the Leman- nice lines- but people build it because the kits or precarves are available and there are darn few alternatives for the non-Hawken builder. I suggest buying the Trade Rifle Sketch Book by Charles Hanson III (a few bucks, softcover, full of sketches and drawings that are full size). You might be surprised to find the "Leman Indian Rifle" in there, listed as 1850-1870, WAY after the fur trade era. The Leman was the most common Indian rifle when they were fighting Custer and being very late in the ML era, more of them survived than other models. So folks figured, Lemans were what everyone used, always.
Most trade rifles ran around .52-.54 caliber, as folks had to tote enough balls that would last them a long period of time. Because they were "trade rifles" they were not customized with different options like larger bores, different furniture, etc. They were off the shelf or off the back of the trader's wagon, and usually came with ball mold, wiper, and blanket case.
Just offering some info- the Lemans are good-looking and handling guns and if that's the main point, or if you want a very late period muzzleloader, make one that fits your needs.