Generally this is caused by either
1--poor ground between battery and regulator
2--voltage drop in the regulator supply harness
3--a bad regulator or more rare, a bad battery cell
4--problems in the alternator or wiring harness problems
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4--problems in the alternator or wiring harness problems: Let's check this FIRST. Unhook the green field wire---the one which goes to the regulator and run the engine long enough to make CERTAIN it is not charging. Hook the green back up and confirm that it begins charging again.
One field wire (green) should go to regulator, the other should go to switched ignition, or "ignition run"
Since this is a conversion, are you absolutely certain that it is wired correctly?
Diagram from MyMopar:
You should have added one more wire
The original green from your field wiring now goes to the "F" terminal on the regulator, and to one of the two field (push on) terminals of the alternator
The added wire goes to the remaining field terminal, and connects in with the original dark blue IGN terminal of the old regulator. This same electrical point now also feeds the new regulator I terminal
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1--poor ground between battery and regulator
To check this, run the engine at an RPM to simulate "low to medium cruise." Make this check first with all accessories OFF and the battery charged up. Then test again with headlights, heater, etc running
Stick one probe of your voltmeter onto the battery NEG post, and stab the other onto the regulator case. Make sure to stab through any rust, paint, chrome. You are hoping for almost zero volts, which would be perfect, the lower the better. More than about .2V (two tenths of a volt) means you have a poor ground condition between the battery and the regulator.
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2--voltage drop in the regulator supply harness
To check this out, turn the key to "run" but engine not running. Stick one probe of your meter directly onto the battery POS post. Stick the other onto the push - on terminal of the alternator which corresponds to blue, or "ignition run." As in the first test, the LESS voltage you read, the better. More than .3V (three tenths of one volt) means you have a voltage drop problem in the harness/ circuit path from the battery to the regulator.
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3--a bad regulator or bad battery
If the other checks above all seem OK, make sure the battery is charged and have it tested. If the battery tests OK, replace the regulator.
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It is unfortunate that you ended up with a "round back" alternator, as the "square back" design is much better. Also, many of the "round back" ones are hack jobs from rebuilders, that is, they have been converted from a 69 / earlier grounded brush to having an insulated brush added.
Round and square back designs:
A typical rebuilder "hack job." This is a 69/ earlier alternator which had been "hacked" to add an insulated brush. The original insulated brush is at 9 o'clock, the hacked brush hole is at 12 o' clock, and the original grounded brush is at 6 o'clock
