I'm running early 340 manifolds on factory Magnum heads. Some guys have had trouble with a bump on the Magnum heads that would prevent the manifold from bolting down flush.

To me, the bigger problem is the floor of the Magnum exhaust port is raised significantly to improve on the flow. This leaves only about 1/8" of common gasket sealing surface between the LA and Mag ports. Remflex gaskets solved my leaks.