There is a adjustable voltage regulator available for the stock alternator. PTE was the last to sell it. You can also have the stocker converted to an adjustable external regulator ...these can be expensive.
I purchased a Powermaster XV alternator. It comes with the adjustable regulator installed and a painted finish. Remember that to charge a 16v battery you need more than 16v. Depending on the model(wet or AGM-sealed) you could need as high as 19v for a period of time.