Gotta love EBay. Just after I wrote about the difficulty in finding performance parts for the Mopar Slant Six, I found an Offenhauser 4 barrel intake for one on EBay. Supposedly it's new, we'll see when it gets here. I got it for $219.50 plus shipping. Most of the online shops I've found charge between $279 and $350 for this manifold, so I'm pretty happy with the price. I also found a complete Super Six setup (intake and exhaust manifolds and carburetor) on one of the Mopar forums, so hopefully I'll be adding that to my stash of Slant Six goodies as well. That's one of the things I like about the Slant Six. It has a good variety of parts to try - if you can find them. For Slant Six intake manifolds, some good choices are :

• Chrysler Super Six 2 barrel
• Offenhauser dual 1 barrel
• Proline Fuel dual Weber
• Chrysler Hyper Pack 4 barrel
• Offenhauser 4 barrel
• Clifford 4 barrel
• Clifford Triple Weber side draft

The Super Six 2 barrel setup is only available used. There are aluminum and cast iron intake versions. The aluminum saves a little bit of weight. I don't know if running characteristics are the same or not. This is probably the best street set up. A Slant Six isn't really big enough to take advantage of a 4 barrel - at least not a street Slant Six.
The Offenhauser dual 1 barrel manifold is a very interesting piece. All of the single carb manifolds for the Slant Six have 3 different runner lengths - a pair of shorth runners feeding the center 2 cylinders, a pair of long runners feeding the end cylinders, and a pair of medium length runners feeding the "in between" cylinders. The Offy dual manifold has 4 long runners feeding the end and center cylinders, and a pair of slighlty shorter runners for the "in betweens." There is also a passage connecting the 2 plenums, which provides for intake heating and also helps "equalize" flow in the 2 banks. I believe this would make fuel distribution with this manifold much better than with any of the single carb manifolds, so this may be the best choice for a high performance or race Slant Six. The Proline Fuel setup takes this manifold and modifies it to hold dual Weber 2 barrels. Probably overkill, but it sure looks cool.
The various 4 barrel manifolds available for the Slant Six are probably overkill, especially on a street driven car. The Hyper Pack was a factory piece from the 1960's. At one time, Clifford Performance was selling a reproduction, but it seems to be no longer available. The Offenhauser and Clifford manifolds are similar. The Clifford manifolds have the advantage of bosses cast into the intake runners at the head, making them easy to convert to fuel injection. It is still available new from the manufacturer. The Offenhauser manifold seems to be out of production, but it's fairly easy to find one on EBay. Expect to pay around $200 to $250 depending on condition.