Somewhere out there on the internet there's an even better website on which the test involves a more thorough exercise in finding out just which manifolds work and which ones don't.
The testers took two early 340 (swept-back) passenger-side manifolds and tested them on a dyno engine, and the results were amazing. Seems as how that driver's side 340 manifold was KILLING exhaust flow, and when they replaced it with another passenger side manifold (which, of course, you can't do in the car) they picked up a bunch of horsepower. I don't remember how much, but it was on the order of ten or twelve, if I remember correctly. Maybe more...
Something to consider: With that kind of unequal exhaust flow between the two original manifolds, there would be vigorous and constant flow through the under-carb crossover, heating that area of the intake manifold up considerably. Not good...
Perhaps somebody reading this may know where that site is located; I lost the URL, like the dummy I am...
Sorry...