273 intake vs. Edelbrock 1406 carb

Just to add a bit more to the mill, also check that your manifold has a flat and true mounting flange. When you tighten down the carb, do you do the cross method???...that is, finger-tight the right front nut, then the left rear, then the right rear, and finally the left front....then tighten in small amounts in the same order to optimum torque. Also, make sure it's a good flange gasket!

Just my thoughts.....

EDIT:...Also, check to make sure there's nothing hanging down from the carb, or sticking up from the manifold, that would cause it to not sit flat on the intake.