Replacing O2 sensor and spark plugs

I have a question about those items.
We just purchaced a 94 lumina 3.1 sedan
I want to replace the o2 sensor and spark plugs. Question I have is how hard is it to "tilt" the engine to access the rear plugs, and doesnt the exhaust mess anything up? Is the 02 sensor back with the rear plugs and exactaly what P/N is the sensor. Napa and others give multiple part #s so I dont want to buy the wrong one.
Local shop here wants 110$ to change spark plugs including parts. That seems high.

Why are you replacing 02 sensor?
110$ is not bad price for plugs and labor.
There is two wish bones on top front of engine to rad support.
Remove them and eng will rock forward.
They make a tool to hook in where the wish bone goes to help rock it forward.
In a pinch you can tie a small chain in there and twist it and pull eng forward.
Also removing map sensor on back od engine will help.
Unless plug wires are new I would replace the plug wires at the same time.
One bad/or weak plug and the 3.1 will go crazy.
MT

Im replacing the o2 sensor because the check engine light comes on after it warms up. Driving on the freeway and as soon as you let off the gas it comes on. 122,000 miles on the car, local mechanic says replace the o2 sensor so I figure while im changing the spark plugs change the sensor as well, and like you said change plug wires too.

You need to check and see what the code is.
Post back code no.
And remember that a code does not say replace the sensor it says check it out.
MT