I am not so sure, I would think you would have an obligation to the company. JMO, but if you were the leadman or forman, then their would be something binding between you and the company. Did you give two weeks notice before leaving ?
I would tread lightly on this one, is there any chance to reconcile with the Boss just to the end of the project ?
If he can prove that the job came in late because you left(providing there is a late clause) you could be held responsible.
Best of luck to you
Wm.Colt