So late time I upgraded I tried using bestPig's tool on Win 7 x64 to create a new nand. Why I ignored the errors and proceeded to flashing, I don't know, but I did and I ended up bricking my 360. Out came the parallel flasher and all that jazz.
THIS time I knew what was wrong when bestPig's tools shouted "image not extracted properly." Win 7 x64. So I used another computer being 32bit XP to make my image. No problems and the operation succeeded fantastically. Proceeded to flash with flash360. Crossed fingers. and. Worked a dream.
Now I just gotta update my DashLaunch and install the official M$ 13599 update to get my full functionality.
I simply wanted to report in. Moral of story! Original nand + bestPig + WinXP = GO. Works great. I was hesitant to use other tools such as coolshrimp or others, since previously I've always used bestPig.
Hopefully this helps someone else in upgrading.