There has been lots of discussion on this, but so far, I don't think anyone has actually done it. I am not aware of any maintenance issues with the 6.0, if built properly, it should last just as long as a 5.0 engine.
I know Talbir had custom head gaskets made for far less $$$ compared to buying new from AMG. Wiesco made a custom piston for a larger bore (details are on 500Ecstasy), so they could probably make one to your spec for the 100mm bore. The cost mentioned was $2k/set... definitely not cheap. Next up are con rods (not sure if anything off the shelf will work besides the AMG rods?), then modifying the crank, and of course boring the Alusil block. Oh, and don't forget, obtaining a 6.0L EPROM chip. The 6.0 cams are a spendy option - I'm not sure if they're worth it or not.
If you do all the labor for engine teardown & reassembly, I'm pretty sure it could be done for <$10k, including the cost of a good core motor (assuming you'd like to keep your current engine stock, as spare). If you're paying a pro shop to build the engine, I expect the price tag would bump up to $15-$20k pretty quick. RENNtech wants nearly $30k for their engine, but that's absurd. I thought Steve talked to Ralph Biase in SoCal and got a figure of roughly $20k for a turn-key engine. Also check with Jono, he has looked into this pretty seriously as well. Talbir never publicly posted the name of the shop which built his 6.xL engine, but maybe he'll tell you privately if you ask nicely?
