Man this would be an all time fight. But if we take both fighter at their prime its still hard to predict because PBF was best at 130-135. At those weight PBF would let his hands go with combos, But RJJ was so fast that he would give him trouble.
I think RJJ would win based on activity and the willingness to let his hands go.
In a mythical fight at a mythical weight I think a prime RJJ would give PBF problems with hand and footspeed. I could see RJJ wining on points. The only way PBF could negate RJJ speed is by using his jab cause RJJ had the tendency of throwing looping LH's and OHR's