Nice match up. I would pick Ellis, I see his solid chin, consistent jab and boxing skills likely seeinh him to a decision, but I wouldn’t count out Mercer.
Ellis is an amazing, overlooked story: Fought at middleweight and couldn’t quite get over against the top guys he faced like Henry Hank, Rubin Carter and George Benton. Then about six months after dropping a decision to Benton he shows up at 176 pounds and a year later is a solid, if smallish, heavyweight and within a couple of years he’s knocking off a string of contenders and taking a version of the world title. As noted, only the top guys like Ali and Frazier beat him in his heavyweight prime and he was a handful for anybody. Even on the decline he was going the distance in competitive bouts against the Joe Bugners, Boone Kirkmans and Ron Lyles of the world. I don’t know of anyone who made the shift from middle basically straight to heavyweight in mid-career and had that kind of success in the modern era.
Really Good Post, the only one I see near the accomplishments of Jimmy Ellis was Floyd Patterson, who started at middleweight.