Is it possible that the Chevy inline six & Pontiac OHC6 could actually share intake &/or exhaust manifolds? The 1-barrel versions look the very same. Bore spacing was the same on the 2 different engines, and it was an opportunity to economize production a bit.
If so, then would the 4-bbl manifold fit a Chevy L6?

go to your local parts store ( with some intelligent parts people) and see if the intake and exhaust gaskets are the same. if they are they will interchange. if not see if they have bolth sets and see what the difference is between the two gasket.