Bonavena would trouble Rocky every now and then, but Marciano would ultimately take the decision on work rate. I don't see a KO happening here.
Boxing is about styles and Marciano would get the best of Bonavena in a brawl.Rocky would love a shootout with a brawler who did not have much of a defense.And Marciano did not tire as Bonavena did against Ali in the last rd in 1970. Marciano was truly Henry Armstrong,with 45 more pounds...Rocky by a late stoppage...
We know fighters from the 1970's and later had weight, reach and size over fighters from Marciano's era. Oh yeah and stronger. So I think Bonavena bounces Marciano out there in a round or 2.