Bugner was a lazy fighter. Rockys two fisted attack makes Joe that much more tentative. Marciano via late round ko.
Bugner had the size and skill to cause rocky big problems but knowing joe he would probably go into defensive mode as rocky hammered away at him Losing on a wide points verdict.
Bugner always lost to his best opposition. He would not start winning against top opposition vs Marciano.
We're not considering eras, but Rock would cop a UD. However a 1975 Bugner against a 1952 Rock might tie up, push away, jab (a good one) and UD Rock. BUT...that's matching up two fighters from different eras. But, given Bugner's resume, I'd still go with the Rock.