Here is a neat trick posted on fordbarn.com by Bill Brock on how to solder the heavy starter brush wires easily. Here is the trick in his own words:
The idea is to heat only the area that is to be soldered! I do this by firmly clamping a heavy wire/cable to each side of the joint with vice grips or small 'C' clamps so there will be no arcing.
Then using a carbon pile battery tester as a rheostat and a 12 volt battery as a power source, you can dial in just the right amount of heat to melt the solder with no flames or hot irons to burn the insulation.
No problems in trying to hold a hot iron in just the right place to transfer the heat. Just make sure the cables are heavier than the joint!
Hope this helps, it works great for me!
Bill Brock
July 2003