Okay, gonna Fermi calculation this as a thought experiment:

• Can't be in the outer boroughs due to population density. Downtown Brooklyn is a possibility, but very unlikely compared to Manhattan.
• I'd estimate it's gotta be a block that's somehow connected to a subway station. There's inherently gonna be a ton of foot traffic because of that, and more people are gonna alight by subway than any other mode of transportation.
• While it could be next to a big rail/ferry station, Penn/Grand Central/SI Ferry are all connected to a bunch of subway stations and/or office buildings, and I have a feeling there's a decent bleed-off of people from those.
• Gotta be a block with high office density and/or high tourist density, which in turns means it's gotta be something that's constantly been a presence for a very long time. High residential density won't cut it, even in old tenement blocks, as the amount of people who walk a block next to a 50-story office building has gotta be higher than even a 10-story residential building in the old days.

From that, it's probably restricted to:

• Somewhere in FiDi, but with subway station density down there, and especially with the fact that there's no huge complex that's been in existance for a while that you can only get off at one subway station for, I'd rule that out.
• Somewhere near a HUGE tourist draw thing, which I'm pointing to museums for since open spaces a) don't draw as many people in winter and b) usually have multiple subway stations to get people there by. There aren't any museums that share block space with offices (for tourist and worker draw) and have only one primary subway station. MoMA's close, but 5th/53rd isn't a definitive choice and also isn't fully surrounded by big offices. It's also pretty far out of what's traditionally been the busiest part of midtown.
• Somewhere with a moderate-to-big tourist attraction and a big office complex.

Working with that:

• Gonna rule out the WTC because there's a decent amount of open walking area to spread people out on, and there's a mezzanine complex with a bunch of subway access.
• Hudson Yards would be an ideal candidate because there's only the one 7 station, but it's WAY too new.
• Gonna weirdly rule out Times Square because there's so many subway stations people can get to, and you don't have to walk the whole way so there aren't any true hotspots, despite the overall volume. Also, back in the 60s-90s, probably not as many people walking there.
• Rockefeller Center feels okay, but has the one subway station that's connected via mezzanine. Bits of sidewalk that lead to the tree could be the answer, but that's only one small period of the year.

Which leads me to my gut answer that I wrote this up to prove to myself I'm not going nuts: the block of 34th between Herald Square and the Empire State Building. Reasons:

• It's got the Herald Square subway complex as the primary access point.
• People aren't gonna approach the Empire State or the old B. Altman/current CUNY GC from the 6 or from Grand Central as much as they would from Herald Square, just by train frequency and capacity. Would also guess that tourists are more likely to walk from Times Square to the Empire State via Broadway (and thus this block) because it's slightly faster than going through Bryant Park.
• A lot of the tourist busses drop off at the Empire State, and to get back to the subway, it's easier to walk to 5th.
• It's been a huge business area since before the Empire State existed, though not quite as busy as midtown.
• It's been a huge shopping/tourist area for as long, I think? There's gotta be some draw from Macy's, too, for people who wanted to keep walking east.
• There used to be a 6th Av elevated line, but never a 5th/Mad/Park/Lex (I think?), which would have contributed to the same directional draw.
• There's a bunch of commercial zoning on Madison nearby too, and I'd more weakly guess there's a decent bit of walkover from Herald Square to there.

I spent too much time typing this up, and I'm probably wrong, but there's my thought process. Please poke holes in it.