289 intake question ("new F4B or RPM Air Gap)

Flip a coin. They're both good. The only difference is the F4B has unequal length runners and probably flows a shade less than the RPM Air Gap. As such it would likely give it a bit more bottom end than the Air gap. And it's probably considered an "obsolete" design by some, but the difference is I'll bet less than 10 hp, hardly something you could feel.

I will tell you from experience that you won't be happy swapping that T289 for any dual plane intake.
I will lay money on that.

I was in your situation, almost exactly, and let people talk me into swapping the T289 for an RPM. It was supposed to give more bottom end, without hurting top end... Wrong.
Engine revved slower, was flat after 3000 (compared to the T289), and had no additional bottom end. The T289 is an extremely mild single plane. It has very good bottom end. If you were swapping from a Victor or Parker, then you might see more bottom... Then again, your cam doesn't start till 3k, so how would you see more bottom end anyway?

My RPM came off within 2 weeks of installing, and I never looked back.

My combo:
302
T289 & 700 DP Holley
66 289 heads with ALL the work done + Chevy valves...
228/235 hyd .520"
10.5:1
Hooker 6901s & FM 40s
11" clutch on wide ratio TL
4.11 gears on 26" tall tires

both DH and ratio gave good advice, confusing i know but bear with me. since you seem to have your heart set on the dual plane intake, the difference between the F4B, rpm airgap, and even the weiand stealth, is going to be but a few hp. combine these intakes with the cam that you have though is going to hurt power through the rpm range. why? because you have a cam designed for the 3500-7000 rpm range, and you want to install an intake designed for the 1000-5000 rpm range, so by the time the engine actually starts getting into its power band, the power band is now gone. you will end up with a badly mismatched combination. can this be corrected, yes. how? best way change the cam to one designed for street/strip operation like the comp cams xe274h extreme energy cam, and then drop on the dual plane intake. but since you dont want to change the cam, then you either stay with the intake you have, or you could modify the new intake you buy, by trimming the center divider about 3/4" and adding a 1/2" thick open spacer, and putting on a 750cfm carb. why the 750? you have a situation here where the smaller carb is creating too much velocity with the dual plane intake, and the fuel will actually puddle in the intake plenum, and create an unstable fuel mixture. sometimes rich sometimes lean. the larger carb while flowing more air, will also have a lower velocity than the smaller carb, and the fuel/air mix will turn the corners better. this is a compromise deal though and will not be as effective as your current combo.

One question--------you stated that the present cam's power range is from 3500 to 7000. Is that what the cam card states, or the actual power observed? You can "feel" a cam "come on" at a certain rpm when it's reached. That rpm and what's advertised is often two different things. There are too many variables involved for any cam company to accurately predict how a cam you choose will act under real conditions. Case in pont: I built a 302 a few years ago with ported E7 heads, Ford B303 cam, 1.7 rockers, Ford A321 intake, Holley 650 DP carb. Power came on at about 1500 rpms and pulled to 6500 I then swapped the above parts for Canfield heads (1.94/1.60 valves the equivalent of AFR 165's) and a Vic Jr intake. Left the cam and carb as is. Power then came on at 3000 rpms and pulled to 7500.
My 331 uses the same Canfield heads, but now with a Ford Z303 cam (228*@.050, .587 lift) and a high rise dual plane repop Ford 3x2 intake topped with 3 Holley 250 cfm carbs. Power comes on at 1500 and pulls to 6500-7000. I don't know what the rpm range is for these cams is supposed to be, but as you can see, the intake and carbs can change a lot in the way it acts. And for you rbhom, the 331 was fantastic running on the single 250 cfm carb with the dual plane, it did nothing you described above.(small carb on a dual plane intake with a big cam) throttle response was crisp and the truck would actually top in at 90 mph on it alone.

I would like to add to my comment:

As soon as I put the RPM intake on my car and saw that it couldn't rev, I put a 2" open spacer under the carb. This helped some.
However, it still wasn't as good as the T289, and the mod ate up all my hood clearance.
After typing that, I realize it doesn't make sense to even offer that up...
It was a band aid that helped, but with hood clearance and air cleaner compromised, the intake still came off the car.

Since you say you want more low end ( I assume for better street driveability) I say use the F4B. They are a copy of the manifold Shelby used, and they work great on a street car. But then again so does your current intake, so it's not like you're going to see a night and day difference either way. If 3.90 gears don't give you enough low end, you're going to have to make a more drastic change than the intake.

Running an Edelbrock Torker-plus cam and ported 289 heads, I swapped from an F4B to a Torker with no other changes only because I got it cheap and I like to tinker, and I actually gained a lot more low-end torque on the butt dyno. The car was much easier to drive in lower gears around town. Swapping from the Torker to a Performer RPM gained me nothing noticeable except for my wallet being $200 lighter.

I keep forgetting to order intake gaskets with my Summit orders, but when I do the Torker is going back on.