The GMC CCKW

Is there some way to figure out if the engine in this truck is the original engine,I have all the data plates and the hood numbers, I've been trying to cut through inches of grease to find the engine number,I'm getting close, it's there a formula for figuring the different numbers?

Unlike Jeeps and a few other military vehicles
I don't know of a list of numbers that references engine numbers and chassis numbers.

John