Really you cant unless you go with a dual alt setup. But. I have heard an seen the HK's run fine on a 12volt setup. Keep it unmodded. or If modded still keep the ohm load to like 2 ohms. You be fine with that. Keeping you volts up. Do the big three Alt, Batt, Ground. And add a couple of good Batts in the back. An as others will say the best thing would be is to get a High Out Put alternator. Not to sure about those Immortals if they really handle 2k all the time.
I have ran my Hk at times with just a 12volt setup but I did kept the ohm load up higher. Going lower than that. chance of harming the amp will be greater.
Hope that helps.