Assuming your first flight isn't very late, of course, that should be no problem. They'll check your passport, etc, at Chicago. At JFK you'll just need to change gates (and possibly do some rechecking of documents at the gate, possibly not), which is not a problem with 1h 20m (even if your first flight is slightly late).

JFK terminal 8 is in an "H" shape, and your first flight may arrive in one half and you may have walk to the other half, but again, it's still all in the same terminal, even if two different parts of it, and you just need to walk for a number of minutes.

It's in the other direction (where you have to clear immigration & customs after your first landing in the USA) that you need a longer MCT.