Can I replace my 3.1 litre and trans in my 02 se with the 3.8 litre and trans and harness out of my 2000 grand prix gt ?

you might be able to do this there are some things to look for check the motor and trans mounts and see if they are in the same location. if they are check the wiring on the motors and see if they are the same which they might be.if you are going to do this its going to take some work is the wiring is the same and the brain boxs have the same pin combination all you might have to do is trade out brain boxs and put the motor in using the wire harness in the car. if it is different you will have to use the wiring and the brain from the car you are taking the motor out of. if you need to take pictures and side by side compair them to each other look for differences

As usual I'm going to play devil's advocate here. As others have stated the project may well be do-able, but you're going to spend a ton of time and a good bit of money (even though you own both cars... trust me on this one) creating one worthless car out of two worthless cars. If you have nothing else to do, and a high level of tolerance for frustration then fly at it. But if you have better things to do, get rid of your two boat anchors and find a GP that's already running. There are lots of very nice ones around for not very much money and you can be back on the road tomorrow instead of 18 months from now. Just my two cents worth.

If you actually take your time and for it right you can do it in a weekend but yes there are a ton of issues doing this type of service sell both GPs for what u can get and just buy one that runs

It’s been two years. I’m guessing he’s got it resolved by now.