Great opinions all around. I agree that Tom was especially nervous about this because everything had to work perfectly if Michael was to pull it off, and as the hour grew late no one
knew where the meeting would be. At some point it would have been too late for Clemenza to plant the weapon in time.

This brings up a subordinate issue here...namely that Clemenza had to go to the restaurant, and presumably order a meal, and then use the bathroom to plant the weapon. I don't see him strolling in and asking to use the facilities without being a paying customer. Assuming that was the case they were already operating on a short timeline, and it is surprising to me that
Sol would not have had someone staked out there in advance. Had he done so that person would have recognized Clemenza and things would have turned out much differently.