I would remove the tin (or have a trusty mech do it for you) and confirm the
head leakage.  As it was suggested it could easily be a bad coolant hose
especially since with the new motor and head gaskets the coolant pressure
could be higher now than it was with your old motor and (maybe) seeping heads
which could cause your hoses to start going bad one at a time.  This happens
alot when new motors or new heads are installed.
Have fun and remember that I'm right up here in South Jersey if you need me.
Ken Wilford
John 3:16
(609)-327-0027