Not sure about the diff ratios. I have 4.3's and I did read somewhere that the auto has 3.9's but I don't know if its correct.
If the car was anywhere near standard I would be tempted to do that but I have done so much to it that I don't think I would get any of the money back. If I bought an auto GU I could try and transfer all the gear across but not sure how much that would cost. I have had the car for just over 30000 km so I have not had my moneys worth from it or the mods yet.
I am not keen on starting all over on another 3.0 litre GU. Too many little niggly bits that need to be fixed/upgraded. Maybe if a nice 4.8 came along..........