I am building a fullstock flintlock Hawken .58 right now, but from a shaped blank, barrel inlet only. 36" barrel, your 42 is going to be heavy. Track's is fully inlet and therefore nothing like the work to build one from the stick, and I've done three of them (halfstock Hawkens) from the stick, and three more from Track's inlet stocks.
I am using the flint breech and tang FHG-16-3 (page 224 of the catalog) which has a plug that screws into the end of the barrel, not the extended "false breech" addition. It is hooked and can be removed, but probably shouldn't be. With the flint breech, the flash hole liner goes into the side of the bore ahead of the breech plug.
With the plug-LRF-16-3 shown in their parts, the flash hole liner goes into the false breech and you have that small powder cavity putting the main charge farther away and complicating cleaning. The plain flint plug is flat faced, not slant like the stock is inlet for. Presume it could be reinlet for that, but if if interests you, ask Track about it. Also, that adds about an inch to your barrel, which is now 43" long.
And if you do get it, check the length of pull before you fasten the butt plate on. I don't know how long it is, but it may be too long. I just got their Kit Carson stock, and I think it was about 14 1/4", too long for me. I cut it to 13 5/8". It is some work to reinlet that butt plate, but better now than having a finished rifle that is not comfortable to shoot, and then you cut it off.
The fullstock is simpler than the halfstock in that you don't have the rib to install, or solder the thimbles, or fit that nosecap. And if you use pins instead of keys, you don't have to fit escutcheons, though they probably aren't necessary and aren't shown in the parts.