I thought the exact same thing after reading the book and seeing the film.

I'm apprehensive to say that the heist never occurred, I just don't think it was really as big a deal as Polisi builds it up to be in the book and film. These were all guys who made their way through the ranks of the mafia via truck hijacking, so to think 5 various low level guys from different families knocked off a truck at some point in the early 70s isn't beyond the reach of what could happen. Some of these guys at that point in the film were still struggling to assert themselves as full fledged members.