Make sure that the ground surfaces and wire connectors are nice and clean.Remember that not only can wiring corrode on the outside and on their connectors but they can corrode internally too.Most of the issues regarding electrical problems are related to the grounds.If you know that the belt is the correct tension it might not hurt to have the alternator rechecked. Again remember that old cars like that are bound to have lots of bad grounds and warn out/corroded wiring etc. Even fuses and relays can end up with corroded etc connection.Don't just look at the grounds, clean them make sure their up to par by cleaning the surfaces etc..Connections can build up an invisible dust that you can only see when you touch it and rub it with your fingers, we call it the invisible grey dust.

Hi so the alternator needs to be replaced first signs of it failing.

I think the problem is with the alternator. Make sure the alternator is well grounded. If that's OK, then I think the alternator is defective. Try putting the stock alt back in.

The alternator isn't working right. You should have 13.7 volts minimum at 1,200 rpm.

i dont believe it is Alternator because the stock alt was in and it was doing the same thing .I was thinking it was thats what led me to put the Mechman in.The Mechman is new and been tested before it came to me.