The drivers side 352 manifold has an indentation to give a little extra clearance to the steering box and flat gasket at the exhaust connection - no gasket at the head (found that out the hard way).
Pretty sure a later 390 manifold will fit in the Squarebird engine compartment and those later 390 maifolds had donuts where the exhaust attached - maybe the drivers side manifold was replaced at some point and that's why it has the donut?
Check out this old thread on the trouble I had using gaskets between the head and manifold.
Hope your New Year gets better from here on out
(like maybe a few nice drives with the Squarebird) rather than wrenching on it.