[olddog]

Where the intake forms the top 1/3 of the valve cover mating surface and the head forms the bottom 2/3, it is possible that these two surfaces are not on the same plane. When blocks are decked and heads are plained, the intake must also be machined to keep all this matched up, and it doesn't alway get done right or even done at all.

I had a 2.9 in a Bronco II (essentially the same design) that these surfaces were off over 1/8". The only time I ever got it to not leak over time was when I pitched the gaskets and filled it with ultra coper. Not saying you should do this or that you have this problem, but you should check it with a straight edge next time the covers are off.