Offenhauser 2x4 manifolds are still available through Summit - pricey though. The high-rise single plane OFY-5698's for the 360/340 are over $425. The #5488's (lowriser for 64/65 small port heads) are a dime-a-dozen on eBay.
www.offyparts.com sells them as well, but they're more expensive...