Doesn't the 4.6 have a bigger chamber capacity than pre serpentine 3.9's????
The 4.6 heads have a 28CC combustion chamber vs 34CC for the 3.9 head.
This is to compensate for the 30thou thicker composite gasket vs the tin gaskets.
AFAIK the heads are physically the same except the 4.6 head has 30 thou milled off. I have milled 3.9 heads with no problems to fit composite gaskets to tin gasket heads and my chambers turned out at 28CC.
Regard sPhilip A