VIN question

[Lennie]

I just purchased a '66 Chevy short bed two weeks ago. Yesterday I was looking at the vin decoder and discovered my vin is for a long bed. I am assuming the cab has been swapped at some point. Even though this seems to be a common practice, does the vin usually follow the cab or the chassis?
I looked at the frame and it has not been shortened. Another odd thing is that the truck is a rear leaf spring suspension. I know chevy's could be ordered with rear leaf springs but it got me wondering if maybe I have a GMC chassis with a chevy cab? Are there any things I can check to determine if it is a GMC? Are rear leaf chevy's that common?

Thanks in advance for your help