Google is your friend ( most of the time)
A 1981 305 SBC (Small Block Chevy) engine that was part of an original equipment system uses a 2-wire, non-heated oxygen (O2) sensor. The 1981 model year generally utilized an early, single-wire or two-wire narrow-band O2 sensor design.
Recommended Parts
You can find suitable replacement parts using these references:
AC Delco (OE Style): Original Parts Group lists an OE style 2-wire sensor, part number L980047, which is a direct fit for 1980-81 GM vehicles with a 305 engine.
Bosch (Universal/OE Type): Bosch offers both universal and specific fit options. The OE replacement parts typically have a 2-wire lead configuration for this model year.
Bosch Oxygen Sensor: 2 Wire, 12.48" Long, Non-heated, Exact Fit
Part # 12028