340 engine block w/no serial numbers

need some help,

I have a 1968 barracuda formula S convertible. Just pulled the engine and I looked on the front left side of the block for the serial numbers nothing but a couple scratch marks ( HAT 51331 ) I think some other than the factory left.
A1.2. was the only marks stamped in the same area below the head. The car was built in Hamtramck in August 30 1967. I've heard that early built engines in 1967 had no vin numbers. I also checked for the vin. on the block down by the oil pan. The casting number 2780930-340-2 validate that the block was 1968-1973.

The engine has heads (3671587 ) for 1973 360. I't looks like the engine had a heating problem. I pulled the intake manifold and notice that the heads coolant passage that match to the intake manifold were blocked off on the rear passages. I mean that the intake manifold at the rear has no coolant passages for the heads. The front passages match up.

I think the engine block is early model don't no, and the heads are late model
and the intake manifold I know is a early model. The heads coolant passage must need a later model manifold with rear passages,don't no.

I this makes any since I could use some advise
thanks chris