What do I need to buy and set up to tune a twin-turbo 408 LS with EFI Live — do I need a custom OS, and should I run MAF or speed density?

Here is what the documentation says. Each point links to its source; tables and names vary by computer, operating system and tuning software.

• P01, P59; LM7, LQ4, LQ9, LS1, LS2; 1999-2007: HP Tuners' 2-bar/3-bar MAP support (the 2.5-bar custom OS) adds a Boost VE table spanning 105 kPa to 255 kPa with better than 5 kPa resolution in the boost region, retains stock VE resolution in non-boost areas, adds a MAP-referenced Boost Enrich table in 5 kPa steps, and runs in speed-density mode only. (HP Tuners)
• Sources differ: Conflicts with simpler '2-bar good to 14.7 psi' statements; the OS boost tables actually read to 255 kPa when paired with a 3-bar sensor.
• GM P01 0411; Gen III LS: A 3-bar speed-density custom operating system allows running up to 30 pounds of boost in this application. (LSX Magazine)
• GM P01 0411; Gen III LS: Converting to speed density required forcing the MAF to fail (0 Hz fail frequency) and reconfiguring the P0101, P0102, and P0103 diagnostics so no check-engine light is set. (LSX Magazine)
• LS Gen III/IV; 1997-2013: A speed density tune is configured so the vehicle fuels from the VE/VVE tables instead of the MAF; on HP Tuners a 0 or 1 in the MAF frequency fail-high diagnostic indicates the car is set up this way. (PCM of NC)

More in fuel, E85 & boost, or build a checklist for your swap with the tune checklist tool.