You need to remember, whether you put 40, 60 or 80 hours p/w into a death march project, won't be recognised. Everyone will be too busy to tally who's working more. By sacrificing more of your free time, you are just working for free for the people driving the project, who may be doing this purely for political reasons. You will also make it more likely for similar projects in the future.
I maintain it's better to not overstretch yourself and let the project fail. Better for your health, and better for the company, so they manage the next project better and have workers who aren't burnt out.