Urban legend. The only functional differences between slant exhaust manifolds regard the choke pockets and emissions provisions. Super Six/2bbl cars had 2 1/4" HEADPIPES, but the exhaust manifolds were the same.
Yep. The biggest differences have been the choke pocket design, the design of that manifold heat flapper thing, and that some later manifolds had an oxygen sensor. The later ones also added stiffening ribs. But any factory slant six manifold will work on any slant six.