How to tell what motor is in a 1972 CMC

[Pat leary]

I just bought a 1972 GMC 2500 from northern California; it was part of an estate sale. It was an old ranch truck with 31,000 original miles that the son had been using for the last ten years or so, AC, power steering and power front disc brakes. The truck has not yet been shipped home to Massachusetts but the question I have is how do you tell what engine is in it. I know it is a big block the seller said last time he had it serviced that the mechanic looked up the numbers and told him it was a 454. My gut feeling is it is a 402, I looked up the VIN on the web and just identified as an eight cylinder. The truck looks like it has never had anything done to it, original paint etc. and with only 31,00 I would think it is matching numbers. If anyone could give me some guidance I would appreciate it.

Thanks, Pat