First of all: Thanks for trying to help.
Second, the neck-seam, RGMage2 found so insufferable, has always been a nuisance in F3/F:NV. But it isn't the problem I have: mine is that I want to port a head-mesh from another game (like Isquall's Femshep mod), which of course doesn't initially fit to the neck-vertices in Fallout.
Third, what RGMage2 did could indeed be a solution albeit a rather lackluster one. One would plunge the new head into the neck as if the latter were a sheath. I find the new seam, which is rather a gap, quite unnerving, so I give this a pass. But it's an idea nevertheless.