Like Mike Brooks said, it should be as wide as the top flat of the barrel where the breech plug tang meets the barrel. Looking at some photos of original Becks, I have notice that he often had a flair to his tangs. This is probably why TOW sent you a wide tang, in case you wanted to emulate this feature. J. P. Beck also sometimes shaped the end of the tang into a decorative shape, but his stock architecture didn't really change over the years.
As far as the stock inlet goes, that should be undersized, to allow you to do the final inletting of the tang and plug. If it is like most kits I have seen, it should be significantly undersized.
Matt