Are you sure about that? The one Clifford 4bbl manifold I have seen does not have the gasket surface on the bottom or the bolt holes to attach a stock manifold to it, plus, Clifford says it can't be used with the stock exhaust either.
I had an Offy on my \6 with headers and a 390 cfm vacuum secondary 4bbl. This combination with a Comp 252S cam made for a good street combination.