[dmjlambert]

Yes, sounds like you have stuff changed around from the factory setup. It may have never worked right since the PO made the changes. If it were mine I would change the alternator out to a regular internal regulated one, either 12SI or CS series and fix the wiring to be correct including 3 wire connections. And also get the temp sensor wired like factory.

First you can probably just start by having a parts store like Autozone give the battery a good charge and test. A fully charged battery should not get killed in minutes on a running truck.

If you continue to have troubles or just have questions, post your questions here and tell us everything you can about your truck, such as what engine and options you have and accessories you have that are not part of the stock setup (such as electric radiator fans and other high-current-draw items and how they are connected), and how exactly is the HEI power wire connected. If you don't have electric radiator fans or other high-current-draw items you should be able to drive the truck for days and hundreds of miles with a good battery and no alternator at all hooked up.