I have done a very little bit of leather stamping, say in the coin purse kit sized project. A project on that scale.... tedious doesn't even begin to describe it, let alone you would probably give yourself some serious Carpal-Tunnel problems with that much hammering. A full body sized anything you are looking at possibly 100 hours of labor, just on the surface design.
The phrase "the devil is in the details" definitely applies here. Perhaps some practice swatches are in order?
For large scale work, a custom stamp is good, but for swatching a regular 1/2" bar stamp should suffice.
Perhaps scoring the leather or doing some other means of affecting the surface to gain the same visual effect might be more time efficient. matte acrylic medium is substantial enough to paint on leather & dries clear. Using matte over a shiny leather just might give the right look.