I never would have called myself a strict KJV only-ist, but I have always thought it was the best english translation out there, as long as you can wrap your head around the antiquated language, which, incidentally, I happen to find exquisite.
As for OP's question, there is a bunch of scholarly writing about how the NT canon was developed, and if you can manage to keep from getting bogged down by modern text/historical criticism, the end result is more or less orthodox.
To give a very basic rundown, (this is still from the protestant perspective, just to be clear) the Church in the first 2-3 centuries decided the canon. There was no one criterion that would put a particular book in the canon, but a handful of criteria that are common themes among canonical books. Most notably among these was apostolic authorship. If it was written by one of the 12 apostles, it was pretty much in the canon. Also to be considered were a book's use in local churches, citations in other canonical books, and theological accord with the actual teachings of Christ and the apostles. For this reason, the gospels were the first to be universally accepted, and a few of the most controversial books included 2 Peter, 2 & 3 John, Hebrews, and Revelation.