Herme,
I'm not sure on the wire colors. The alternator case should have the numbers 1 and 2 cast into it under each tab (DA plug). The plug only goes in one way and the wire for #1 is smaller than the wire for #2.
The smaller wire (#1) will attach to any switched source of power, like the key switch. The larger wire can have a terminal crimped onto it and attach to the big BATT lug.
If you can't find any markings on the case, the #1 wire/terminal is the one nearest to the large BATT lug.
Because the truck has already be swapped to 12 volt, it was probably run that way for awhile and everthing should be OK, fuel gauge included. If you have any doubts, come on back and ask more questions. Good luck.
Stuart
