I thinbk it's commonly accepted that there are two points in the Washington Heights area that have 260' closed contours. One is in Bennet Park (Fort Washington between 183rd & 184th) which has a stone plaque proclaimimng it to be the highest point in Manhattan at 265.05' (but the topo shows no spot elevation, just the closed contour). The other is in Fort Tryone Park (about 1/2 mile further north) which also shows a 260' contour on the map. That one may have been "enhanced" by man made landscaping, it's hard to tell.
Here's my report: http://members.bellatlantic.net/~vze2h6gy/papabear/NYC_5_borough_highpoints.html#day3
There are many others out there.
Check the County High Points site: http://cohp.org/ny/new_york_E.html#New%20York
for a few other reports.
I've never heard of Long Hill (and I live in Manhattan).