200R4 all the way if you don't wanna cut the tunnel. That said if you don't mind cutting the tunnel, look at the 6L90 tranny. 6 Speed auto with great ratios so you can rear 410 no problem.
Are the 6L90s a viable swap yet? I heard the the electronics were still a problem. I've seen a few of the TCI 6Xs in action. Not bad, but a bit pricey. (Seen a few failures, too.)
I've installed a few GVs and they are nice. You definitely know when they engage. I do feel they are a bit application specific depending on your needs. My big concern which nobody seems to address directly is if you have a big hp engine, don't you usually use a high stall converter? So, if you have a tail OD that puts your engine speed significantly below the stall of the converter, won't that build heat in the trans? Sure, you can add a bigger trans cooler, but it doesn't address the slippage.
For the money, I would go with a modern OD auto with a lock up converter. My opinion is that it would give better benefits.
I do have a friend that abuses the heck out of his GV and it has not complained at all. (It's mounted to the back of a 4L80e).
