I don't see it. While Spinks by no means had a bad chin (it was very good at 175), I don't see him handling Marciano's power all night. And it's not like he had the footspeed to evade Rock all night.
His footwork was a bit bizarre, but it got the job done. He was able to evade qawi for most of their fight
I love Qawi, but Spinks tamed him. He put enough hurt on Qawi that Qawi didn't bum rush him, and even seemed a bit spooked. Marciano couldn't be tamed, he didn't give a fuck. His face could be falling off, and he'd still pressure guys and just hit them anywhere until he found an opening. Plus, unlike facing Qawi, Spinks has to worry about true fighting-ending power. Qawi wasn't the type to end a fight with one punch like Rock could.