1974 big goodie

I wonder if any of you people know anything about the early '70s industrial line 6500 series GMC trucks. I have a 1974 with a BIG block V8 a 5 speed tranny with dual pto's. It is a big rig wrecker. Anyways I need to know how to identify what the engine is. I think it is a 454. I'm not sure but i think GM kept making 427s in their industrial assembly lines intil 1976. Anyone with any info would be much greatly appreciated. Thanks