As I mentioned above, the 337 is not as tall nor nearly as wide as the true MW window. I think they did this on purpose so a well ported 906 size port could consider it as an option.

Before sending my manifold to Larry I scribed the port match to my heads and gaskets, there was a good bit of width and some roof height grinding to get it there.

I went back and saw some dyno sheets from a 572" 4.5 x 4.5 (I'm sorry can't remember whether it was Firefighters or Old Schools but compared side by side their 2d vs the 337 each with a 1" open spacer the peak results were pretty 'close' 713.9 @ 4600 to 721.2 @ 4500 on torque and 697.3 @ 5700 to 705.2 @ 5800 on HP, my motor is "only" 517 and the 2D I had was deeper ported, so you can probably see why I wasn't in any real hurry to swap. the 337 was more, just not that much more. I also think (to make it true "apples to apples" fair fight) the 2D could have been tested with a larger carb (the 337 had 1050/4150 and the 2d only had an 850/4150?). If anything a dual plane generally wants MORE CFM to compensate for the divided plenum, a single plane (as we all know) allows each port to access all four barrels of the carb. I've been running a 1000cfm/4150 Pro systems on mine with both manifolds.