If you are going to need AVGAS, it is a good idea to call the General Aviation Terminal well before you go, because the AVGAS truck is not always right there. Also, the General Aviation Terminal does not directly sell the fuel. You will need to get your credit pre-approved from the vendor right before he pumps it. Im no JFK expert, I have been there in as small as a 310, and it was not a big deal. The people at the GAT were actually very pleasant and helpful when I have been there. Look at Airav.com, and call the GAT if you have any questions.
When I have flown into JFK at 11:00am ish, it has not been terribly busy.