1958 Ford 861 Tractor

Here is some short reading for you regarding changing the system from a positive to a negative ground:
http://www.spritespot.com/Downloads/Positive to Negative Ground Conversion.pdf

What was the condition of the tractor when you first got it? Was it running? Did it have a 6 or 12 volt battery in it? Do you remember if it was hooked up to a neg. ground? Does it have an ammeter? If so, was it operating normally (reading a + charge when running)?
I think you need to get the tractor manual and a test light and just track down where the juice stops flowing. At least with the tractor everything is easily accessable.
C