Before installing ANY alternator, clean all its mounting surfaces thoroughly, and apply ELECTRICAL grease (not dielectric, or thermal, or chassis). If the alternator case isn't fully grounded to the block (and then continuously to each battery's negative post), it can't fully charge the batteries.
Similarly, the block ground cables & battery terminals must be clean & tight. Peel the insulation back from the battery terminals to clean all the corrosion out & polish the mating surfaces, and install studs & nuts instead of factory-style bolts for superior clamping force.
.
