At his best, I'll take Olivares by stoppage late in an excellent fight. His inside work and pressure would be the key, as he would be able to box well enough with the taller Zarate, but I think he could break him down in the later rounds. If off his game, Zarate is capable of starching Ruben or stopping him on cuts at some point perhaps.
I just don't think Zarate proved himself against elite competition enough, not nearly as much as Olivares anyway, even if he was more consistent in his prime. The competition could've played a part in that as well.
Maybe Im placing too much emphasis on Zarate and his debilatating power and better reach, although if Olivares can close the gap and get inside I can easily reason to why Olivares can win.
A past prime Olivares was outboxing Arguello at FW prior to getting stopped in the 13th, so I think it's safe to say we shouldn't underestimate his boxing ability.
.............Having started to really see a lot of Olivares in the past year, I find him to be one of the most overlooked greats in terms of skills. He was known as "Rockabye Ruben" because of his power and killer hook and body punching (he was Mexican, after all), but he was a canny, fluid boxer as well, one of those rare talents that could pressure you all night long, but do it smoothly in a patient, calculating manner. I have to say he impresses me more than Zarate. I could see Zarate winning by KO as well, Ruben could be gotten to, but my gut tells me Olivares is able to work his way inside and take the lanky Zarate out of his gameplan and stop him around the ninth. Great fight either way.
Olivares he worked the body better and had better boxing ability and had same power for me, it be a great fight but I see Olivares ending it in the championship rounds.