I think someone has you duped with some fandangling verbage calling them "factory dual exhaust" manifolds. The fact is any manifolds can be dual exhaust as each manifold comes off each head, so in esccence all V-8 manifolds are actually "dual". The exhaust either stays seperate keeping them dual or joining them together making them single exhaust. Make sense?
There are rare factory nailhead manifolds that had "D" shaped ports. As a matter of fact, I think there are at least about 4 different configurations for the exhaust manifolds as you go through the years. Russ Martin posted all of them with photos and casting numbers on the Yahoo newsgroup a while back. Or you might giving him a call, and he can help you.
Just as another thought.. are you sure they are 401/425 nailhead manifolds?
More importantly... what is this going in? There may be some fit issues around the steering box/and or suspension. As far as performance, why not headers anyway?
Erik
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