Fenech was just one tough determined hombre and as skilled at the prime JJ was I can't see him keeping Jeff off of him forever.
Jones' weak chin lets him down. Fenech gets inside his reach and ravages him. Fenech by mid rounds ko.
Two things... Fenech wasn't exactly a hard puncher. He needed oodles of volume. Jones controlled distance pretty well. And Jones had pop. Fenech is not going to walk in and KO Jones. He would have to do steady and persistent work to achieve that while not taking too much in return.