The 362 FMW that everyone loves on the 6.0 uses a 62mm FMW compressor, 68mm turbine, and .83 a/r housing. So if you're familiar with how those run with a specific injector, the 64.5/70/.91 is going to be slower to spool but have a much stronger top end and cooler EGT's due to the big jump in compressor flow and lower back pressure from the 70mm turbine (with the same injector). The 70mm turbine wheel really seems to work well on the 6.0L and I love the way it works in my Duramax. Great middle of the road exhaust size option between the 68mm and the 74mm (found in the standard 366).
The 64.5mm FMW wheel actually flows about the same as a cast 66 Borg wheel, but due to its size and weight will spool better while making pretty similar power. As long as your drive pressure doesn't get crazy from tons of fuel of course. Most of the 64.5/70 combos have been built with the .91 a/r, but a .83 or 1.0 could be used as well...