Honestly, I do a lot of in and out of BOS and I think you might be better off buying the ticket originating in JFK and taking Amtrak to New York. I am always very nervous connecting at JFK from the BOS flights.
If you take Amtrak, you can change at Penn Station to the Long Island Railroad; take that to Jamaica, and then the AirTrain to JFK. From arrival at Penn to arrival at the Delta counter at JFK should take about 45-60 minutes. The plus too is that if you have a broadband PC card or BlackBerry, you have data coverage for about 99% of the trip.
Depending on the time of day, you can get an Amtrak ticket for about $75.
I'm not sure you can do BOS-JFK in coach and then JFK-FRA in business on the same ticket as it will marry the segments at JFK for purposes of fare calculation. So even if you sat in coach BOS-JFK it would probably still price out as a business fare from BOS.