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A developer’s lucrative proposal to build a high-rise between two Brooklyn Heights co-op buildings is threatening to tear the close-knit community of neighbors apart.

Denizens of the 33-story building at 75 Henry St. have the opportunity to score big bucks by selling a slice of their land along Pineapple Walk to an unknown real-estate firm so it can erect a 40-story residential tower there. But the residents of a 31-story co-op on the other side of the tropically-named lane say the deal would literally and figuratively build a wall between the two groups who have lived as one happy hamlet for decades.

“We’re not kids, but I think it would change things between us,” said Shelly Holtzberg, who lives in Cadman Towers, which hosted a packed public forum about the sale on Wednesday. “Right now there’s really an open door and we’re even talking about sharing services for both buildings.”

Cadman Towerites organized the gathering to come up with ways to convince their neighbors to reject the mystery developer’s $75-million offer for the land, which is currently home to the beloved Park Plaza Restaurant and several stores.

The deal would net each 75 Henry St. unit at least $120,000 according to the co-op’s board — but the resulting tower would also block views from both buildings, cast a shadow across the area, and bring an influx of residents to an area that has an overcrowded school and recently lost its hospital, claimed critics.

The 75 Henry Streetians have until Jan. 15 to cast their votes, and a majority of owners will have to vote “yes” for the co-op’s board to pursue the deal.

The opponents say they know they’re asking the residents to turn down a big windfall, but hope they’ll consider the impact their decision will have on the whole neighborhood.

“We’re asking them to give up significant financial security that this offer would bring to them, to their children, to their family, and to their future,” said Peter Bray, who is the executive director of influential local civic group the Brooklyn Heights Association.

At least two 75 Henry St. residents pledged to vote “no” at a previous meeting, but one brave resident told Wednesday’s crowd he intended to say “yes” — stressing that it was just a vote for the board to explore the offer, not a final handshake.

“I would be in favor of it,” said a man who identified himself as Gil and said he has lived in neighborhood for 60 years. “But all we’re doing is looking at the possibilities — we’re not making a commitment.”

Both co-op complexes were built in the late ’60s as part of the Mitchell-Lama housing subsidy program, which created below-market-rate housing aimed at middle-class families. Cadman Towers remains in the program, which means units there are relatively cheap — a WNYC reporter claimed she scored a one-bedroom for $38,000 in 2010 — and are only available to people earning below a certain amount (currently around $97,000 for three-person household).

Shareholders at 75 Henry St. opted out of the program years ago, so it now functions as a standard co-op — a one-bedroom there was recently on the market for $750,000 — and the owners are now able to cash in on the developers’ offer.

Reach reporter Lauren Gill at lgill @cngl ocal.com or by calling (718) 260–2511.