ground polarity is determened by look at which post of the battery is tied to the chassis.
gasser fords pre 65 left the factory as positive ground ( and 6v ).
diesel fords left the factory as 12v and negative ground.
you can setup a ford genny and run it as positive OR negative ground.
If you have an alternator on it.. it becomes less clear. the bulk of alts are 12v neg ground. but you can get 6v alts.. and positve or neutral(isolated) ground units.