I am somewhat surprised that the fit issue is "not enough drop", but I am unfamiliar with a Mears and the Martini-Henry, I know only by the action, but I believe both are European, and that may account for higher combs on older guns. It has been my experience that too much drop is the issue, but that is with American guns. Shaving the combs might get you what you need, but modifying those guns to shoot them more may run into durability issues after the fit is accomplished. You may find it difficult to buy a durable gun with enough drop, modern guns being higher in the comb, especially those of Asian make. A older Wingmaster might give you an affordable and workable option.