Looking forward to the pictures. This sounds right up my alley.

Initial thoughts: that serial number is too early to be an actual Springfield Armory Sporter so that lines up with your observations about the barrel being swapped.
Sporter parts were available though and it sounds like someone put one together.
I agree with you that this wouldn't be a "scrounged parts sporter," though. A nice stock, a sporter barrel, a Lyman 48, and a G&H scope mount...someone put a fair amount of cash into this project!