Ue, going by the pictures and the axle code (which appears to be the original - except you're right, it should be a posi), I would say the car started life as either a L35 (396/325) or L34 (396/350). The 3.07 axle was the "standard" axle for both of those.
The original 12 bolt, the big block heater box, the 5.5k tach all point to a big block, so congrats!
Ed