as said.. it was originally positve ground. sounds like yours is negative ground IF the negative post of the battery is tied to chassis.
I notice you said 'ground' of the battery was tied to chassis of tractor.
that's BAD nomenclature.
a battery and charge devices have polarities.. ground is a 'reference point , or even plane, if commons are used.
saying battery negative is MUCH less ambigous than saying battery ground, dealing in situations where either may be used.
now you need to determine IF there is a reg hidden on the steering column.. or if you really have an alternator.. or a genny that has been gutted and made into an alternator. there are specialty shops that will make your genny an alternator... etc.
usually.. gennies are cylindrical and longer than they are in diameter. an alternator is about as long as it is wide.. usually aluminum.. etc.
see if the charge poutput stud on your device runs right to some point where it hits battery hot.. or if it goes thru a reg first.