It seems to me that a coolant system pressure test, cylinder pressure test, and a cylinder leak down test would be helpful.
Regarding what may have gone wrong, did you check the block deck for straightness? Cylinder heads?
Did you save pictures of the block head? I have heard of case where ppl continued to drive even after evidence of cylinder leakage was occuring. This caused pitting of the block material between the cylinders. Lacking a smooth surface, the new gaskets are not able to hold a seal.
If this were my car, I would want to rule out a leaky intake manifold.
I also agree it may be easier to pull the motor.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.