My intake manifold leaks like a sieve

As Scooby-Doo would say... 'Ruh-roh' LOL

As said in post #8, time to back up and do some test fitting to see why the gaps are wrong at the back. With that size of leak, the coolant probably never got up high enough to show similar bad leaks up in the front.

Q2: Are the new heads re-manufactured or j-yard pulls or ???
Q3: Were the alignment sleeves in place on the head surfaces of the block to properly align the heads?