The cross-ram (413 Chrysler F, G 413 RB; 383 B, MW) was specifically designed with runner length to add torque based on the engine's power rage (2,800 and 5,500?). Small plenums, small carburetors, and high velocity in the runners. Since the left and right manifolds are separate castings, an original manifold ($$$!) will bolt to any B/RB engine, shorten and re-attach the runners to your calculations.
I spent many happy hours in an air-conditioned 300F convertible back in the day, and its "manners" never betrayed the plumbing under the hood.
Too long for big cams an good heads.

What would a manifold be with runner length tuned for 7,000?
Tunnel ram - the runners just extend vertically.

The Weiand 6-Pack offers some ideas, see my comments here: http://victorylibrary.com/mopar/weiand.htm