8 volts? Holy crap, Batman. What I would recommend you do now is run a jumper between the alternator output stud and the starter relay and redo the readings. If the voltage difference from the alternator to the starter relay is now near 0 volts you have a poor connection in the wiring from the alternator to the starter relay. Biggest question at this point is do you have the stock ammeter? The stock charging circuit goes alternator to bulkhead terminal, bulkhead terminal to ammeter, ammeter to bulkhead terminal, bulkhead terminal to starter relay. If the jumper between the alternator and starter relay worked, then you have a poor connection somewhere in the above path. If you have eliminated the ammeter, then you will have to determine where a poor connection may exist based on how you modified your system. The other voltage between the alternator case and body should wait on what you find out with the above tests.