It took me some time to come around to the long barrel rifles, but I am ready. I'm building my first flintlock, an American longrifle, and I opted for a full 44" long barrel. The key though, is that I went with a swamped barrel. I've only just began, but I think this will be a sweet shooting rifle, and not just off a log. They are supposed to be one of the better offhand shooting rifles. It's not as crazy as it sounds, as this will have an OAL only 8"-12" longer than my favorite wing shooting shotguns.
The problem in the OP's case is the barrel is a straight barrel, and thick to boot. I'm not sure why they recommended 1 1/16" wide barrel. I think most 45 calibers are 13/16". I believe 1:48" is a standard twist for 45 caliber, so that should be a problem. Anyway, you fixed your problem, and made your rifle work for you.
@Good Cheer, what is that? It looks like a TC Renegade, with a brass Renegade style trigger guard (which I've not seen before), a TC New Englander lock (or at least the hammer), and possibly a custom barrel, or at least custom sights. The TC New Renegade Jaeger? TC NRJ?