If your A is equipped with lights, the generator F post should connect to the field resistor terminal on the back of the light switch.
However if it does NOT have lights, the genny F post should connect to the cutout's FIELD terminal.
Simply grounding the generator F as you have it now does work. However it causes the generator to charge at its highest rate all the time.
This is not a problem if you use the tractor only intermittently. But if you run it hours at a time, the constant high charge rate will quickly ruin the battery. If this is the case replacing the cutout with a voltage regulator is highly recommended!