I (when a used tractor dealer and farmer) converted several 6 volt gennys to charge a 12 volt battery simply by using a 12 volt Voltage Regulator and NEVER had any problems. The reason is a genny doesnt "necessarily" know what its output is to be, the faster it spins and/or the more current you pump through its field windings the greater its output (that's one function of the VR to regulate output by controlling fiedd current) . YES its still designed for 6 or 12 volt use WELL DUH but the above theory still holds true.
NOTES before anyone has a calf now:
I'm NOT saying that's engineering correct or the proper way to do it. I'm ONLY saying it works and I NEVER had any trouble.
It also might depend on what Voltage Regulator is used such as those having both Current and Voltage Regulation control relays versus those that use a Combination Voltage and Current control relay.
There could be a chance when this is done if the field coils draw too much current they may overheat but I NEVER had any problems.
I'm NOT saying this will work on ALL Gennys and ALL Voltage Regulators. Theres a huge difference in 2 brush versus 3 brush gennys and voltage regulators (2 or 3 coil etc). The ones I used and worked were Class A 2 Brush units.
THE CORRECT AND ENGINEERING PROPER METHOD is to have a shop convert the genny or just use a 12 volt genny and VR
You ask: "but are Farmall and JD generators that different?"
They are alike in that both are Class A so the above still applies
Hope this helps, I advise a non electrical person to do it the correct and proper engineering method so you dont risk damage to the VR or Genny.
Hope this helps