"It seems to go down into the deck and..." is the part that led me to believe it was keel stepped. Dunno Cal's plan, so there it is.
Ya might need to do some investigation on just how the mast base is set. Perhaps a PO had some leaks and liberally applied goop around it?? Sounded almost like a mast boot... another indicator of keel stepping.
Seaward and some others use mast/deck hardware that can be used as a hinge when stepping. If yer gonna hafta chew off the old crud surrounding the mast, it'd pay to change out to hrdwr that will do the same.