I have a 73 240z with no electric fuel pump. I just installed the ZXP plug and a 280zx 60amp alternator. The new alternator appears to have a cut out on the back of it. Do I need to unplug the one that was bolted to the external voltage regulator?

Thanks

Post pictures of both alternators so it is clear what you are talking about.

The cut out in attached to the right side of the external voltage regulator.

Here you can see an object that looks like the cut out attached to the back of the new alternator.

Here is a clear picture of the cut out with the external voltage regulator removed.

FYI, most people here will refer to that as a condenser. You only need one. Go with the newer one. Also verify whether or not your new alternator is internally regulated.