1991 sierra 4.3 o2 sensor location

Hi All recently purchased 1991 gmc sierra 4.3 withno o2 sensor . Looking for the location to install. Their looks like a plug was put in near the #2 cylinder on exhaust manifold. Thanks

Looking at the Autozone website, it makes a difference if it is 2wd or 4wd. 4wd is passenger side exhaust manifold. 2wd is under the truck on the passenger side before the cat.

//2000CAYukon

91 was OBD1 and liokely only has one O2 sensor. It will be between the cat and the manifolds.

If you have the new part it should be pretty easy to climb under the vehicle and find the part that matches?

Thanks for the help. 2wd ,the exhaust was changed no cat , no sensor , no nut to screw sensor in. I will install near pass side exhaust man .

Thanks again