It may surprise ya to try both the undersize and the right size manifold with and w/o the lip step flow turbulizer at head entry. I lost carb bowls and carb and manifold fasteners until I used rubbery type thick-ist gaskets at the seams beside some heat blocking also keeps some tension on the fairly lightly torqued fasteners with pot metal flanges. Practice makes perfect and Cdo's are perfect example to practice and practice and practice on. You might also find extra length of intake path is nicely noticed too.