Try using the 8 volt battery in the older Ford tractors.  NO other changes necessary.  Will work with most I would presume.  Added on 6/08:  sounds like the general public would rather have a long and drawn out mechinal procedure. Or you can go to the farm store and get an eight volt battery.  Put it in the tractor. Dont do anything else.  It will start fine in cold weather. Dont do anything else. It will charge just fine.  Dont worry about overheating the starter (as with a 12 volt).  Dont put a voltage restrictor in before the distributor. And yes I have used the 12 volt change when I perfer not to have the generator on the tractor.  For the average non pulling older tractor I put the 8 volt in and dont do anything else. And yes I did pull with the 8 volt replacing the 6 volt and I didnt change anything else.  can you believe 0 for 10 on something like this?