Neill used Track's Hawken Flint Beavertail Tang, Plug- BT-16-3, which is solid and does not unhook. The one I used does unhook, and Neill recommends it also. Track told me their stock in their fullstock Hawken kit, EHF-16-M1, would not work with the Flint Breech and Tang #Plug-FHG-16-3 because of inletting. I think because the tang is narrower on this plug than on the "Flint Hooked Breech #Plug-LRF-16-3" they use. But there is an easy solution. Get their J&S Hawken fullstock, not inlet for lock, #Stk-HF-16-M1 (or M3), 1" barrel, 7/16 rod hole drilled, for barrel to 37" long. It is also not inlet for tang, trigger or buttplate, which is what you need. If this is what you want, call Track and discuss this with them to be sure it works as I have said. The last thing you want to do is build this from a blank. I have done half a dozen or more this way and gotten a few years older doing it.
Track says of their Hawken Pistol Flint Hooked Breech "Drilled for a powder chamber, you may enlarge this cavity, but it must remain smaller than the bore diameter, to prevent entry of a cleaning jag and patch, which might become stuck."