1969 C10 frame swap

[TJB70]

I'm a new subscriber today, and excited to be here! I've been on the hunt for a 69/70 C10 swb fs for a bit. I purchased a 69 two weeks ago. I inspected the vehicle pretty thoroughly. Not that I'm against a long WB conversion, I was paying for a 115" frame. Everything appeared to be factory or factory replaced. I knew the vin on the door post and glove box did not match the vin on the frame, but wasn't concerned.
Ok I'm starting a novel here let me get to the short of it. I'm trying to identify what my frame is from originally? I've searched through countless threads trying to identify with little success. The PS mount area has no recess or raised bosses. I'm thinking it's possibly a 67' frame but not convinced. Could a 63-66 C10 frame been used?