There's probably 20+ HP in working the Lower plenum to "tweak" the 2 weaker runners on the 2D, the upper plenum runners already flow ~355-360 in MW form. With the radius work the ports will all flow about the same, look up the Roger Helgessen (Dr Air) article. Once you do those tweaks you have a dual plane capable of up to an over 700HP that gives unequalled throttle response and (most importantly) roll-on part-throttle torque that makes a 500" motor ALWAYS feel like 500", from wall to wall off idle to redline. ot knocking the Indy single, it's a great HP maker at WOT, but being a street driven car spends ~90% of it's time below 4K and at part throttle and below 75 mph, the 2D is just an overall more efficient torque builder, especially for a 3500+ pound mostly street car...you'll see I've run the single on a couple 493's, it's more of a 'light switch' , the torque is either 'off' ( yeah, it's good, but by comparison, maybe not as good as it could/should be) or 'on' ( but 'really' on).

For the street : I always visualize running the Motor on a road race type powerband, not a drag strip. The 'overall enjoyment regardless of road conditions means more to me than a few extra Hp 'bragging numbers' up top....maybe I'm just getting old. But I'll give up a couple tenths in ET if it would give me back seconds off a lap time....opinions vary I guess.