Engine and parts mixup?

[1969C10shortbed]

I have a '69 C10 short bed step side with a 250 in it. However, I've been interested in rebuilding the 250 and have been researching parts. As I began, I began to wonder if the engine in the truck is the right engine. I ran the casting number on the side of the block through the stovebolt registry and they don't show a record for it. After tons of searching, I finally found what I was looking for, and it doesn't sound good, nor does it sound correct. The casting number I have on the block is 473483 which through searching, seems to be a 250 from a 73-86, but the truck is a '69! The truck has always been in the family, and my mom (it was her Dad's truck) says the engine was never changed, but when the truck was bought in '70, the engine was rebuilt. If it was rebuilt in '70, a '73 and later engine would not have been available. The reason for the rebuild was because it was an airforce work truck before it was purchased by my grandfather and my uncle. I'm racking my brains over this, and I don't know why. Should I go to the local pick-n-pull and get a period correct 250? Or just rebuild this one? Without much information on this block, I know I'm going to have a hell of a time getting parts for it. The local pick-n-pull charges 200 for an engine, so I don't mind going out, removing about 10 bolts and pulling an engine. I don't have any desire to swap in a V8 right now because it's not a hot rod. I have a turbocharged neon for that. Another concern that was brought up while I was researching was that the carburetor does not seem to match anything either. It's a Rochester MonoJet with the carb number of 17056014 which while searching for it, popped up as a '70 truck carb. I really just want a matching engine combination for the truck so that I can drive it for many more years to come as I slowly restore it. Any help that anyone can offer would be great.

Thanks,

Chris