Positive battery terminal to earth was the original setup on the Dexta.
If it is still equiped with a generator, this will work fine, providing the generator is properly polarised.
Hook up any battery charger as normal, positive to positive , negative to negative.
Same for jumper cables from a truck or another tractor, just don't let the vehicle frame or bumpers touch. The other
vehicle likely will be negative earth and you may start an unintended fireworks show (arcing between the two vehicles).
The old forum has a lot of posts on this subject. you can still search them.
here is one that may be helpful.
http://www.fordsontractorpages.nl/ubb/F ... 00016.html