Dextron 6...do we know who stocks it? if it's available it would be rude not to 
Well that depends. Its worth remembering that the Dex 2 pas fluid was upgraded to Dex 3. But was downgraded to Dex 2 again due to leaking pas pipes.
Its up to you of course, but who is to say that similar issues may or may not arise by using anything other than Dex 3...? We know Dex 2 ain't good.
The assumption that a newer Dexron fluid will be better, or even equal to what's known to work is just that. An assumption. ...the mother of all opps ups!
Dexron is a gm trade mark and was developed by them for their own purposes. All Dexron fluids supersede the previous, and are said to be suitable. So there's no reason why Dex 6 wouldn't work, in theory.
But do you want to be first to try? I know I wouldn't.
Let's say someone pops up on here and says "I've used Dex 6 with no problems". How long has the fluid been in? The life of the car? Ok, go for it. But that's not likely. It left the factory with Dex 3. So any upgrade fluid will be less than the age of the car and by definition unknown to be suitable for the life of the box.
Dex3 might be hard to find, but it's known to work. Doesn't cause issues, and we regularly see 200k miles plus out of autos with 3 or 4 fluid changes.
Dex 3 is known to be problem free.