1988 Ford Bronco alternotor not charging correctly

I have an '88 fullsize bronco with a 302 and latley have noticed the voltmeter on the gage cluster is low when I stop in gear. The battery light also comes on for a moment. When I take off the volt gage pegs at 18! I checked with a meter and got 13 volts at idle and 17.5 at 2500rpm. I put in a new battery and a voltage regulator and now only get 12.2 volts at idle and at 2500tpm with a meter? After I turn off the engine the volts go back up to 12.5? I will take the alt. Down and get it tested, that is all that is left right? The parts store only had one voltage reg. In stock! I will try another one of those also!