I really don't think you want to do that. Having too little breech plug tang is not something I have ever worried about. Assuming the tang goes down in the wood deep enough (1/8" give or take) at the end, just file the excess tang off at the top to match the curve of the wrist. You should remember that the breech of the barrel and the recoil lug of the breech plug bearing against the wood as well as the breech plug tang is what supports the barrel. The wood forward of the lock does not much more than hold the ramrod. That is the reason the old time gun builders (and myself) didn't (don't) generally bother to closely inlet the more than the last 6" or so of the barrel. It simply doesn't make any difference, at least not to the type of shooting we do.