I can't take measurements until I head up to check it out. (the used intake). The seller related to me that they were 'in the neighborhood of 15/16"x1 15/16"' I do not know the plenum dimensions. I also do not know how thick the runners are, to enable one to hog them out...
I can always get some 1"-2" carb spacers if needed.
I will get you this information when I go to check the manifold out. The guy wants $550 for the intake plus two 'just rebuilt' Carter competition 500cfm carbs. I figure worst case, I can sell the carbs and replace them with some thing smaller.