The T-56 is what I'd consider, they are however in high demand. They are expensive, but not as expensive as a ZF by a long way, also more common and cheaper than the supra 6 speed. The T-56 would require a custom bellhousing which I'm pretty sure is available through CRS, you would also have to modify the tailshaft. The clutch might also be fun, I know it is when they fit them to falcons.
Now I'm gonna throw around a few numbers because this is something I've thought about a lot. An interesting thing about the commodore version of the T-56 is that its top ratio is 0.5:1, generally used with a diff ratio of 3.46:1. The top ratio in a VN auto is 0.70, manual is 0.73 with a diff ratio of 3.08:1. 0.5 with 3.46's in the arse is pretty tall even for a stock LS1, so for a 5 litre it might be an idea to put in some shorter diff gears if you want 6th to be useful. Oddly enough using 4.11 gears which are easily available would mean that the revs at 100km/h in top gear would almost exactly the same as with the standard box and gears. To me that sounds like a recipe for plenty of fun, especially with a couple of mods thrown at the engine.
Anyway, what I've just outlined is my plan for drivetrain once I've got some cash. It could probably be done for around the three grand mark if you shop around for the parts and do the work yourself. If you intend to pay someone else to do the labour, three grand would easily get you a T5 conversion with change and thats about it.