Also, take a look at your generator/alternator. A generator should have a tag stating the voltage, while an alternator will usually be 12v but there are 6v alts. You could also have a 6v system converted to 8v (generator only) the best shade tree way to tell is to round up a 12v bat. with a couple dead cells, get the truck started with it somehow and see how much the generator tries to charge to.
