You will have condensation and residual in whichever deck is not used commonly. What unused intake volume does, simple physics. ALL dual ports do that. From normal reversion that all engines do. You will always get a reversion spike when the intake stream then hits a closing intake valve, some bounces backwards. Having a manifold that has part of the volume in downtime allows that to stack up in that space. One that is active all the time keeps that residual swept out.
Hot Rod Magazine March 1981 shows the correct way to mount the carb, can't help it if people want to throw away free power to make something easier, I watched it happen (as well as corrected it) at the garage all the time. We worked on and built hotrods of all types. Seeing an Offy on a car made it easy, one of the first things done would be yank that for an Edelbrock if one existed.
The Offy guy saying one thing means nothing, when you take people who could not make a Quarter Pounder 3 times the same way right and then promote them and move them further out into society, well what you get. I hear of lots of retarded information coming out of the speed parts makers nowadays and why Offy is still (not) killing the world with sales now.
The Offy cutaway manifold used for nationwide ads shows lower deck used but it is only half the volume of the upper.
The whole dual port thing was a sad lackluster gimmick when Edelbrock was stomping the crap out of them with repeat product like 180 hi-rise, Torker, Tarantula, the list goes on and on. Edelbrock actually redesigned to come out with totally new product never seen before every time. Offy takes the same crap 360 Equaflow and then adds a cheap divider to say they have totally redesigned which was a copout, any existing that were converted to dual port had the same shortcomings as the unconverted part. Most like the 2.3 one have no true 'runners' as such, they are simply convoluted and constantly varying space that does nothing as far as engine tuning through length or diameter. '50s company still stuck in the '50s even now. Why any Edelbrock manifold will almost always surpass an OFFY by a LOT in most back to back tests. They did back then as well. In the case of our AMC engines like 50 Hp. difference between a Torker and a supposedly high rpm manifold the Offy 360. It couldn't touch the Edelbrock.
Your car, do as you will.......... .........wait till you see the mileage set up like that, it should suk using the shorter bigger runner and low down in the port 99% of the time. Longer smaller runners increase part throttle fuel mileage, more physics. Looking around on the web at others including V8 ones showed differences, some had lower on primary, some had higher on primary and bigger area runner or smaller runner on primary, expected nonsensical application like Offy often does.
After looking at enough V8 dual port pics something insidious even begins to show. They set up the primary to flow WORSE on purpose so that the driver perceives fantastic gains as the secondaries open up since the primary performs so bad. Or catering to the seat of the pants for advertisement there rather than giving you more all around power doing what would have been more logical. Can't brag nearly so much doing that. They are actually killing achievable low and mid range on their own product to bump up top end only. That's just plain cynical to me.
Luck.......... .......