If your tranny goes out, Id make it a manual. But I think boosting an auto is a good idea, especially if you hit the strip. I dont know much about turbo, but i would think that the auto would help keep the turbo spinning, so it wouldnt have to re-spool each time you shifted. Since you have so many miles, dont run a lot of pressure. Stock LS blocks can handle 11 on stock internals, but I wouldnt run more than 8, especially if its a DD.