Mopar Performance / Direct Connection used to offer Slant 6 sand cast aluminum 2 barrel intake manifolds years ago. Did they sell many? Where did they go? I looked at them in the catalog for years and when I finally went to order one (yes, they were still listed in the catalog at that time) I was told they were no longer available. There is an early version of one on Ebay that is ending in a few minutes.

Can I weld a 2 barrel adaptor on to an aluminum one barrel after I machine off the 1 barrel mounting pad?

They probably did not sell many of them, no. For what they are, they were very, very expensive. I haven't measured to confirm my suspicion, but it's my sense that the blurb in the MP book about "larger runners" was just as much hooey as the blurb in the same MP book about "9:1 pistons" that were nothing more than ordinary plain ol' stockers. I have one on my car; found it at a swap meet.

Weld a 2bbl pad onto a planed-down 1bbl intake? Sure thing! You can even pick whether to put it parallel or perpendicular to the engine axis—parallel has several rather large advantages. If you do this, take note of the factory carb mounting pad angle before you machine it off. It is taller at the rear than at the front, to angle the carb front-downward relative to the manifold's top plane when the manifold is installed (you can also reference off the heat riser mounting surface, but these tend to be pretty chewed up on used manifolds).

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