Mickey Walker's performances against bigger men is well documented, as he held his own against pretty much every significant contender from the late 20s-early 30s. Would Rocky be a step too far?
I wouldn't pick anyone smaller than Marciano to beat him with sheer skill, like Walker would have done, aside from maybe a prime Ezzard Charles. Aside from maybe him, the only way is to blow him out or just outmonster him, Langford has a solid chance of doing it, but not Walker.
Walker's run of foe at heavyweight was simply incredible. However the very best men of that division, would have been a reach to far for him, in any era. Max Schmeling showed us that.