what scotty said.

Never trust it. With the Nissans it can be pretty close since they are MAF cars. The mafs have a tendency to compensate alot better than the MAP sensor cars.

What most people do is run the CHIPPED Rom tune ecu with a SAFC to fine tune it.

What you will want to do is put the ECU in the car, then call someone like Mainstream Performance and do a simple $50 3 pull session on the dyno to check the a/f ratio.

if its safe, let her rip, if its dangerous then you know you need to have it tuned and not to beat on it,.