i have found most factory installations are as you have said, 2-3 1/2 [and a couple at 4] degrees downward slope on the china walls [as well as said valve covers] which makes the carb mounting surface level.
now with that said, some aftermarket intakes have more or less slope on the carb mounting surface.
you may, or may not, be able to level out the carb mounting surface by shimming the transmission mount a bit.