Pains me to say it but I'd probably bet on Broner to win. At 135, his power was there and he had size and speed too.
I like Johnston to win, BUT I do think Broner would be competitive. He's quicker, rangier and more athletic than Stevie, and at 135 he still had good pop. But I can't pick Broner. Stevie had a higher ring IQ, and Stevie had more intangibles. I think Johnston would just outwork him/outfight him down the stretch.