If you miss the Delta flight to BOS because your incoming AF is late, or you just can't get through customs and do the transfer in time, you've lost the value of the DL ticket and have no recourse. I'd do one of two things: chance it and resign myself to going into town and grabbing Amtrak to Boston if I don't make it, or cancel and buy the later JetBlue ticket (which you will also lose, however, if your AF is really late).