First off, welcome to the Forum. You've come to the right place for any information you might need with your building project.
As for the kits you mentioned, the "best" one for you depends on what you want to use the gun for.
If you just want a rifle to hunt with, the Cabela's or TC is probably a good choice because they are lighter than the Great Plains Rifle.
Most folks find the GPR shoots roundballs better because of the depth of the rifleing grooves and the twist rate.
The Cabela's and TC both have a faster twist that is more suitable for slugs but they will shoot roundballs well too.
Oh, while I'm mentioning it, there is also a Great Plains Hunter, but it has a fast twist that is not suited for roundball shooting at all.
Of course, all three of these are basically production rifles which were left unfinished. There is some sanding and finishing required but most of the hard work is already done for you.
If you buy one of these and want anything in the way of help, or just to answer any questions you might have, post them here and you will recieve more help than you ever dreamed of.
A word about the "kits" from places like Track of the Wolf, Pecatonica River and Muzzleloaders Builders Supply:
These are more of a box of rough parts than a factory kit. Nothing is finished except the lock and even it requires locating, drilling and tapping the holes for the lock screws.
I don't mean to say these aren't quality parts, and they will make excellent rifles but there is a lot of work and knowledge envolved.
For example, one of the three kits mentioned as possible candidates would take about 10-30 hours of work (depending on how slowly you go) to finish.
A kit from TOTW, PR or MLB can easily take 160 hours or more to build.
The Hawken from one of these companys is amoung the hardest to build correctly.
Let us know what you decide and keep us posted on your guns progress.
zonie