Remove the valley pan and clean both the heads and intake mating surfaces very thoroughly and then take a set of feeler gauges with a .0010 to .0025 gauge and set the intake on the heads and rock it around to locate it and then see if you can get your thinnest feeler gauge to go between the heads and intake surfaces at the bottom and top of each manifold corner Write down what you find on each corner, top and bottom
Let us know what you find