Here is the 67 headrest internal parts.
https://pgclassic.com/products/810buk-67kit
Here is the 68 kit
https://www.classicindustries.com/product/1969/dodge/dart/parts/MD2047.html
They may be available at other resellers so check around.
IMHO
Only putting a factory OEM manifold on with gaskets and bolted down to check for alignment is a valid test.
Also as a control looking at the fit between OEM head and OEM intake with gaskets and bolted down is needed to make the claim that the head's ports are off.
OR OEM engineering...
My OEM 67 one family Dart did not come with the black cup that locates the hard board cover. It may have been a Plymouth thing or 68 up?
Maybe it's the wrong one?
Head rest were an option in 67. You could get drivers only, pass only or both
There is a kit you need to buy to retrofit the seat to accept head rests. The 67 headrest is unique one year only. They changed dramatically in 68. Iirc the rod of and the spacing is different 67 to 68.
You probably...
Another way to go to find leaks is with a smoke gun and air pressure.
If you seal a fan to a window and suck air out of the car. A friend with a smoke generator outside will pinpoint where water can get in. You can pressurize the car and do it in reverse too
https://www.musclecarparts.cc/1967-1974-mopar-a-body-glass-channel-weatherstripping---curved-like-factory-requires-cutting-%26-guide-holes-25375
Lots of sellers sell this same kit so find a seller that has a good price and service.
It requires the removal of the door glass and vent window...