[jtheory]

It's worth emphasizing... the paper trail must show you're doing everything you can to warn the company that this project is currently on track to fail, and (as much as you can illuminate it) what the costs to the company will be.

Back up your reasoning as clearly and simply as you can.

From the perspective of your employers: because they're paying your salary (and others as well, including half your boss' salary dedicated to this), for a failed end result -- firing you as the scapegoat doesn't leave them with nothing, it leaves them with far less than nothing; all of that time and salary lost, plus the costs of replacing you (whether because they've fired you, or because they've burned you out entirely). Plus costs to reputation, which harms client relationships and hiring both.

Being able to say "I told you so" later doesn't really help anyone; but if you can suggest a better path (drastically reducing scope, extending or segmenting the delivery timeline, or even canceling the project), then you should be able to get someone to listen.

If your boss really grasps where things are going (and that this isn't just "the usual developer griping"), then he'll get credit as well for saving the situation, and you both benefit.