VIN/Engine code help

[sbd4de3]

So, I bought an 86 SWB and have decoded the VIN to know that it came from the factory with a 4.3L V6...

But, I have a V8 in the truck. The owner "had no idea" it didn't have the stock motor in it. (He was selling it for a friends niece).

Anyway, the stamp on the block says 10424UFS which decoded tells me: "1980 350 4 bolt main".

But, the VIN overlay stamp says C8A366203... I can't decode this one.

So, my question is this: Do I need to decode the vehicle VIN on the block, or am I just trying to do more research than I need to do?

Thanks for any help! Also, if you do find any info, could you post the link so I can find it too? Not that I won't believe you, I just have this funny need to verify everything with this truck now... (Thanks to friends and their nieces)