Frankie, welcome to the forum!
You state you want to build from a kit, not a blank. I looked at Pecatonica's Hawken kit online. I see that it is pictured with the lock inlet started. If you do pursue this, then you would need to inlet the lock plate first and then inlet the barrel and tang (as stated earlier, temporarily connect them and inlet as one.) You might be able to order the stock without the lock inlet started at all, in which case you would inlet the barrel/tang first, then the lock plate.
I built a full stock Hawken from a Track of the Wolf kit. Mine was flint and fulllstock, so it's not going to be quite the same as a percussion half-stock, but there are similarities. One thing the Pecatonica picture appears to show is a comb that doesn't look to be at the proper angle for a Hawken....my Track kit was the same...too "straight stocked." Net...as previously suggested, study pictures of actual Hawken rifles to understand the architecture.
If I were to get ANY Hawken again, it would have a tapered octagon barrel to reduce weight up front vs the straight octagon most kits come with. It probably would not reduce overall weight, just move it further back for better balance since the tapered barrels are usually 1 1/8" at the breach for larger calibers. I would also change my key retainers to pins vs the screw thing I dreamed up.
While more pricey, you may want to look at the kit offered by The Hawken Shop. These come with tapered barrels. This link has lots of information including a downloadable construction manual you can review.
Hawken Rifle Kit - The Hawken Shop
While not a percussion half stock, my build photo album (with explanations if you double click on any pic) might give you an idea of what all is involved in a Hawken kit like this.
2014/15 Hawken Build | Flickr