put in 98 montecarlo engine

I have a 97 3.1l V6 grand am. my dad who builds race cars had a buddy of his put in a 98 monte carlo 3.1 V6. The first problem with this car is that if the a/c compressor is plugged in it will blow the fuse its on and a few others. this problem was present before the engine was changed. the problem now is that the check engine light came on a few weeks after the engine was replaced. I took it to get a diagnostic and the man told me there were 1o different codes on the car and that it seemed like every system in the car had a error. he started my car and said it seemed to be running fine though. I parked it because the brakes were acting up and plus i didn't know what was up with the engine light. Now there is a puddle of trans fluid under the car. Can anyone tell me whats up with this poor car.