Most routes via JFK/BOS would have 4+ hrs segments in Y or Y+ equivalents...
I would put the other 4 in Y and take the J seat.
Norwegian. Direct from LGW to SJU. Gotta leave the 23rd though. Premium seat is 160 degrees and not too flashy that you'll make your fellow travellers jealous.
GBP 538 in Y (with bags)
GBP 1400 in 'J'...