Yeah, first make sure exactly what you've got. Get all the date codes and casting numbers off the block and research them. If it is in fact a 421 that's been punched out to 4.180, don't panic just yet, but do be prepared to find a facility that can sonic check this specific block to find out exactly how much metal thickness is in all the bores. We're talking mass produced parts here, and there were variances. Could be this one's ok, could be not --- but the only way to know for sure is to have it measured.
p.s. Even if it IS too thin, if it's something special to you and you want to use it anyway, you could always have it sleeved. Of course there's more cost involved, but it's not impossible.