I am converting my PT 81 to 12 volt positive ground how will that affect the gas gauge and the amp meter ?
12 volt positive ground is really rare. It should have no effect on the ammeter, but you will need a voltage reducing resistor on the fuel gauge. I'm not sure what effect the positive vs negative ground will make. Move slow on that one and wait for someone with more skill than me. When I bought my 39 P8 in September (2015) it had already been converted to 12 volt negative ground with a one-wire alternator. The previous owner completely replaced the instrument panel, so the fuel gauge wasn't an issue. Every light bulb had to be changed to 12 V, and you can see in the pic that I have no radio. The original dash is long gone, but I wish I had it. The old instruments were a work of art. As most of the members of the forum will tell you, there is nothing wrong with 6 V as long as your connections are clean and you use thick enough wire. I have had to rewire most of mine P8 and ran all 16 or heavier. I hope to convert it back to 6 V some day. Best wishes on your 39. Both the 39 cars and trucks are beautiful.
A 6 volt 1 wire alternator works fine with perhaps only a bracket to block modification as the size is different from the generator.The alternator should come with an internal regulator.