Prime Ribalta fights the Louis who fought Carnera. Could Ribalta make it to the end or do the unthinkable and beat a young Joe Louis? Remember, Louis from this fight was only 196 pounds.
Obviously Louis is much the better fighter but MAYBE Jose could make it to the end of fight and lose wildly on points. Maybe he gets stopped late in the fight as he did against Tyson? Jose seemed tough and durable in that fight.
It's hard to say how long Ribalta could last. He did give some of the big names of the 80s and 90s a tough time, Smith, Tyson, Witherspoon, Dokes, and Holmes. And Louis struggled with some of the Burn of the Month Club fighters, some of which were probably in modern day on Ribalta's level. The Burns lasted everywhere from round 1 to round 15. I think you have the most logical conjecture though. I don't think Ribalta would last any further than the eighth.
It is hard to gage, but I feel that Joe would probably take his time in figuring Jose out, and get him in that time frame. Ribalta was a tough nut in that late 80's, early 90's time period.