Marciano by stoppage Berbick is fairly durable but maybe could be a tad vulnerable against punchers, he was stopped in 2 by Tyson and in 1 by Mercado.
It is going to come down to the same question as always...could Marciano have handled a 6'2" 220 heavy? I believe he could due to his power and volume, and point out that such men existed in his era, they just weren't good enough to earn a shot. Berbick was a pedantic fighter, and I don't think in Marciano's class.
Trevor Berbick though strong, durable would not be able to handle the constant pressure of Rocky Marciano, especially in a 15 round battle. I think that Marciano would have been too relentless for Trevor, constantly banging him about the arms and shoulders, Berbick would melt under Rocky's constant assault by round 7.