Thanks for all the help, gents. The combo of thoroughly cleaning the breech and plug, oiling them up, strengthening the hold of the vise, and using a longer lever-arm action snugged the two marks right up. You guys rock!
I was able to move forward and start inletting the tang, where I'm afraid an errant slip of the knife cut away too much from one side. I'm thinking I can fix this with a plug of scrap glued in and sanded flush. But before I go that route, any better ideas or tips?
Sorry for being the high-maintenance new guy... I do very much appreciate your help!