If it's a small block heater box, the rear end is original and the Muncie hole is original, then the car started life as either an LM1 (if the car was built before January 1st, 1969) or an SS 350. However, the 3.07 ratio was the optional "Economy" ratio, not the "Standard" ratio on both engines. The standard ratio for the 1969 LM1 and L48 with the Muncie M20 was the 3.31 non-posi.
Ed