You can't mix PMA components with electromagnet type alternators. They live in different worlds.
So, you cannot use the DUC rec/reg with the stock alternator.
If your stock alternator is working, just keep using it. The stock rectifier and regulator that you have on your bike are both very rugged and have survived quite well over the years. However, they are both of older design and are somewhat crude compared to more modern units. You can continue to use the stock rec. and reg. if you wish, as long as they fully charge your battery. To fully charge your battery, you need to see 14.1 volts at the battery terminals, when you rev up the engine to approximately 3000 rpm.
If you don't see 14.1 volts, then the simple inexpensive option is to change over to a VR-115 and a new 3 phase rectifier, which will make the charging system work 100%.