My bet is that the bit near the upside down L shape (a bunk?) is at least glued on - could even be deliberately solidy fixed, I see no reason why not as not against the hull. Plus the bottom even if intended to have a gap with hull would have had gravity to cope with when originally fixed in place.
I would be minded to take the bulkhead out in two (or more!) pieces! Make the problem smaller! Before cutting (pretty much where access allows) could screw a couple of battens to the bulkhead, remove before
cutting! but when bulkhead out would then allow you to reassemble and ignore the cut (and air gap!).
In any event although old bulkhead to be used as a pattern it won't be exact - will still have a few hours of fiddling to fit!....especially as likely it was fitted before the deck
was bolted on!
Just a thought (before bulkhead comes out - what about marking the hull etc edge say 2 inches inside all the way around? (use a small block of wood), that way even though the edges will be chewed up during removal
you will know that the "Good" edge for the new bulkhead is that line plus 2 inches. That way could be less shy about carrying on with the blue cut line! (as long as inside your 2 inch line, or make the line 3 inches deep!)