Whoa pahdnah!
Let's start at the beginning. You said "I'm working on a 39 GMC and the battery is connected - to ground."
Do you mean that the positive terminal is connected to ground, and the negative terminal is connected to the starter? If yes, this is correct.
The first thing to do is to replace both cables with "double ought" cables to carry that 6 volts. If the cables were replaced at one point, they were likely replaced with 12V cables which are only half of the guage required to carry the juice needed for 6 volt systems. These new cables will have to be made up by a truck garage for you and will be as big around as your finger...or more. They are big honkin cables. Typically, the positive and negative terminal posts are different sizes and most "positive" cables are set up for starter install. You need the negative cable to go to the starter and it should be at least a two ott cable.
This will make a world of difference right off. Take (00) the positive cable from the battery terminal to a bolt on the back of the bell housing. Then take another (00) cable from the back of the bell housing back over to the frame to set up the ground side correctly. Then take a third (00) cable from the negative terminal to the starter. This will make the engine spin fast on starting and will get the old electrons a zooming along a super highway instead of down a windy cart path through the woods.
If you reverse the polarity, you'll create a host of new problems...all unnecessarily.
