The V8 GMC used in 1956 trucks was the 316 and it was basically a Pontiac engine. The seller is either pumping you full of bs or he doesn't know any better. They never used Olds engines in the smaller trucks.
If the engine number you saw was stamped on the lower right side of the block it probably started with 316, if in fact it is a 1956 GMC engine.
As far as value, it would be worth more if it was a ½-ton shortbed but it still is worth $3000-$5000 if running, stopping and looking as good as you say.
