Actually the 1976 Cadillac Fleetwood was a carryover from 1975. So there is no way to know if this is the correct vehicle, however, since Hoffa disappeared in 1975 this vehicle would be the appropriate vintage for the final scene.
When Red Bennett is standing on the truck outside the Kreger warehouse, he is talking into an RCA Type 88-A microphone. A newspaper in a later scene shows that this event took place around January 1935. The 88-A wasn't manufactured until 1938.
In the scene where Hoffa, Billy Flynn, and Bobby Ciaro are preparing to set fire to a laundromat, Hoffa tells Ciaro to stay in the truck's cab and keep the engine running and in gear. This makes no sense, since Ciaro would have to keep the clutch down for the entire time it takes Hoffa and Flynn to set the fire. It would make more sense for Ciaro to keep the transmission running in idle, then quickly put it in gear for the getaway when Hoffa and Flynn are about to re-enter the truck.
In addition to Billy Flynn having a revolver that is constantly cocked-uncocked, as the revolver is brought to a vertical position as Billy is uncocking it for the final time it somehow changes from a modern (for the time) double action revolver to a single action Colt. All of this happening within the 90 degree sweep from horizontal to vertical.
When Jimmy and Billy torch bomb the building, they create a blast so intense, it blows out the passenger side window on Bobby's truck. Yet in the following shots, the window goes from being intact, to being shattered.
During the huge street fight at the RTA headquarters trucks loaded with men roar into the fight. The first truck that enters tips over. If you look closely at the back of the truck where the men are standing, as the truck turns over, you can see safety bars for the men to grab onto so they don't fall out of the truck.