I assume the truck has been converted to 12 volts already? I'm not aware of any 6-volt alternators other than some very spendy custom-built ones. If the system is 12 volt already, the simplest alternator conversion is the Delco 10-SI type which was used as original equipment on everything GM produced from the late 1970's through the mid-1990's. Try looking for an alternator for any Chevrolet 350 from 1976 to 1983 or so. From 85 on up, some GM alternators had metric threads and pulleys for serpentine drive belts.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!