Why not get a pre-inlet lock mortise? This can be a blessing or a problem, depending on your needs, abilities, and how things work out.
It can be more difficult to inlet a lock into a pre-cut mortise than to do it from scratch. Stock suppliers have to make the plate inlet undersized a little else the lock plates would be loose in the inlet (customers usually file and polish lock plates; they are castings, not 100% uniform). When inletting into a pre-cut mortise, things can get tippy.
Next potential problem with a pre-cut lock mortise is the barrel and touch hole alignment with the pan. If the mortise is pre-cut, you have no leeway. Maybe the barrel has a longer or shorter breechplug, etc. Maybe you want to thin the stock even more and want to set the barrel in 1/16" deeper. Can't do it.
Last, you have no leeway to really customize things. Maybe the build you envision or the gun you're copying had the lock tail at a certain angle to the wrist, etc. Or maybe you want to customize the lockplate shape, etc.