What are the positives/negatives for doing this?

Read the Mopar Direct Connection engine book
I can't remember but think it was to add a little top end--dual Planes are under rated folks think they will not make HP like a single and that is mostly correct but the TQ down low that is given away with a single plane is considerable
I always relate this tale--while at Indy Cylinder head one PRI week Herb McCandless brought a high compression giant roller 360 CI aluminum rod for dyno test with the then in development Indybrocks--it made a little over 600 HP with single plane but still managed over 584 HP with an air gap dual plane--I raced 1/8th all my life and always had a dual plane RPM or such with maybe a spacer
Most LD 340's you find these days are cut down center divider because the Mopar book said do it It will not hurt the intake

If you have the room just add a one inch open spacer. You'll get the same performance improvement without killing the resale value of the intake.

Too Cheep to by a HOGAN.....lol

If your tune, especially idle quality and staging for a converter, is sensitive, removing the divider will reduce your vacuum signal. Typically dealt with by increasing idle spark, more pump shot, advance the cam 2 degrees, etc.
A 180 with no divider has worse C-to-C mixture distribution, and different plenum volumes between high and low levels.
As Andy says, add a spacer and try it out.

Dual planes are a mess with distribution at higher RPM EGT's on dyno usually reveal scary differences--this can be fixed easy with staggered jetting--but requires egt / dyno to do the best
I always found that 1/8th mile the RPM or Ld340 worked for best 60 ft and low TQ--singles will MPH a tad better and distribution is a breeze but in normal car 1/8th mile small blocks ( like your normal 318-340-360 combos)the dual plane is a plus
Bigger cube or way good heads--higher RPM range and all 1/4 mile we use a single
spacers are miracle working devices but the main thing they do is just add Volume
Volume and matching volume to CI is the key

Think of the intake as a beer cooler--8 thirsty cylinders want to reach in and get one--the bigger it is and the more that are in there means no cylinder reaches in an empty cooler -- my analogy anyway

Removing the divider on a dual plane manifold is to help balance the A/F between the plenum and runner as well as cylinders and decrease the chance of having lean and rich cylinders.It makes the idle smoother,increases volume both at midrange and top RPM.This is similar to the old Diamond Eng.X-rams where the interconnection between the left and right side was dramatically increased buy raising the roof of the interconnection or cross-section to balance both sides and give each side the benefit of increased A/F volume as will as similar A/F mixture.

I know of a member here who did some testing of this very thing on the dyno.
His isn't running the typical street/strip combo though.
Maybe he'll chime in.

His testing included a 6bbl set up, as well as a Holley SD.
The dual plane was the Indy 440-2D.
My recollection is that he started with the 440-2D at full divider height, then started cutting it down in a few steps, testing at each point...... And showing a power gain at each step, finally reaching the point where the divider was completely removed, which made the most power.... On that combo.

I've sold quite a few of those manifolds as part of a top end package for BB strokers, and I usually cut the divider down about 3/4" as the starting point.

this can be fixed easy with staggered jetting

IME this may be limited to a specific throttle opening (vacuum level) or RPM range.

Now there's an analogy I can relate to. My RPM is box stock and since I have zero hood clearance to add a spacer, I've thought about extending the notch that Edelbrock casted in all the way across the plenum.

..... Watching in "learn" mode.

B G Racing, that was a kick ass answer. I can only add for younger/newer members to the
Mopar world that MP has said to use this mod with cam lift of greater than .550. Which you have to understand that this is a read between the lines statement and you will have to remember that most of the cams back in the day when this was written were pretty big on duration at these lifts. AKA, Do this mod with a big ass cam only.

The mod was a "cheat mod" for the dual plane rule in the middle car program. As I was told. I can see it. It's still a dual plane!

I did this to my old LD-340. It helped the top end significantly! Mid range as well. I ran a cam as small as 216@.050 intake duration and it was still OK. Though it shined really well with a big cam.

I've never tried it, I used to just radius the divider at the top, and run a 2" spacer (Open or HVH transition spacer)
One thing many people don't do is up the carb size. We've run 850-950 carbs on Air Gap and Stealth intakes with great success on 340-365 engines.

Ok, I saw an intake on the ole craigs that had the divider cut down. I would put on a street engine, 318, not a race engine. I would want the torque at the lower RPM.

Has anyone tried to put a divider "back in"?

You are splitting hairs--divider gone will not hurt a 318 or any other engine any noticeable amount IME

years ago, i made a divider for a single plane intake from 1/8" aluminum plate, and it was installed by milling two slots in the carb mounting surface. this locked it into place when the carb was installed. i experimented with different height plates, but i don't remember the results. ["oldtimers" disease...... ] you could possibly use this method to "replace" the divider.

Ha,
I added a 2" carb spacer and cut slots in it so I could add a divider to it this way the plenum divider in the manifold was extended all the way up to the carb.

This was an edelbrock LD4B intake running a blow through 650 carb and procharger @8psi.

Been almost 20 yrs ago so I"m not sure why I did it.
After reading this I should have cut it out.

I did notice I had staggered jetting in the carb though.
Front back and side to side.