Ok so what I got is a beretta I been racing for 4 years its dirt track derby time racing similar to rallycross or stadium trucks. Anyways the car has took a beating over the years looking to replace with a 98 cavalier. My current motor is a 3100 with 3400 head and intake swap and some computer tuning on the dyno. Im looking to put this in the cavalier. My question is mainly about the wiring can I use the computer and motor harness out of the beretta with the harness in the cavalier. Remember this is a race car no gauges or radio or anything need just needs to run and move. Also to add this was a obd 1.5 that I had done a obd2 swap on already so its not my first rodeo with re wiring. I just want to comfirm I have all the right stuff to make this work before I got and buy a car.

Also to add the computer is from a 97 Lumina that has been tuned for the 3400.

You can absolutely use that computer and wiring. If your gauges aren't an issue, than it's a pretty clean swap.

How much re wiring would I have to do as far as taking both harness apart and wiring them together.

If you just want the engine to run, you'd only need to splice about 8-10 wires, mostly the pink ones that correspond to the correct power states (off/run/park)
If you want to get more of your gauges to run, you're looking at double that.