Vern FWIW, you're probably in overload with all the stuff that's been thrown at you, but here is my take on your question regarding "best twist"...might as well get it out and take the flak, but it is "my take".....
But first I will say that I also have a T/C .50 with the same twist, along with tens of thousands of other T/C owners, and to the best of my knowledge there is little doubt as to the accuracy obtainable with this rifle with this same twist...loads may vary, to include different powders, different patch thickness, and different lubes, and on and on, but there is never any argument about about the guns ability to perform.
Now when you ask, what is the best twist? That becomes a hoss of a totally different color.
Personally, I don't know of any best twist for any given caliber, as I have seen a bunch of what I thought was weird stuff perform flawlessly...an example of that is the .50 GM 1:28 twist with a round ball and 60gr FFg. (not the FFFg powder recommended for the caliber, nor the "minimum" of 1:48 twist for round ball, and not the conical bullet this barrel was expressively designed for.) but it shoots surprisingly well.
Another example might be the lowly .56 caliber smooth-bore with a round ball...mine shoots a good as I can hold, so why would I ever want better, and yet many folks feel they are inaccurate.
Bottom line of all my gibberish is Capt. gave you some good information.
What you want, and what you intend to do, has everything to do with what you choose for a rifle and what twist that rifle will have.
Just don't overlook what you have right now.
I have also long heard and read that the 1:48 rifling machine used by Hawken in St.Louis was the only machine available at the exact right time, and I have long believed that story. I have believed it mostly because it made good sense to "make-do" with what you had during that time frame, and I don't see Sam Hawken as being any different.... Was it by design, or the luck of the devil?
You be the judge. You pick the version of the story you like best, but never think of it as the 1:48 twist is somehow the wrong twist, or a one-twist-do-everything-twist.
It is a good twist, and it has served tens of thousands of black powder shooters well.....
Is a 1:66 better for round ball? Most likely.
Is a 1:28 better for conical? Most likely.
Is it the best twist? Probably not for specific tasks, but a better twist has been slow in coming.
Uncle Russ...