On a late note, Montauk, NY, USA really should not be a city (apologies to all 4,318 people who live there). Long Island, NY is a somewhat unique place; as of 2020, while the island has is quite dense (Nassau County is the 22nd densest county in the US out of over 3,000 counties, while Suffolk County is denser than all fifty states), yet there only major city is New York City, and only other two* cities are not big cities (Glen Cove has 28,365 people, while Long Beach has 33,454 people). Instead, Long Island is endless suburbia; it's nearly 50 miles (80 kilometers) from New York City's eastern border to the first rural area, so over two and a half million people are scattered among villages and hamlets (and two small cities).
There's no perfect answer, so my recommendation is to declare a town a city (even if it technically isn't one). Hempstead is the largest one (793,409 people as of 2020), but since that's completely covered by Vic 1836's New York City sprawl, the second largest one, Brookhaven (485,773 people as of 2020) would likely be a better choice. Alternately, instead of having a city on Long Island, scatter a few spare houses across it as map doodads (and remove them if NYC reaches that far).
Unfortunately, I don't have 1836 statistics available for specific villages/hamlets, but since Suffolk County had only 32,469 people in 1840, making Montauk (which has ~1.5% of Suffolk County's land) into a city would not be feasible.
* Port Washington -- or, for you Great Gatsby fans, East Egg -- tried to incorporate itself as a city in 1930, but failed; even if it succeeded, it would be on par with Long Beach.