Manual mode SB800
The guides are given for 100ISO. M1/128 with diffuser is given as 1.5m. Will that be 3m or 6m at 400ISO?
It will be 6m. The math behind Guide Numbers automatically accounts for the Inverse Square Law, so going from the published ISO 100 to your desired 400, is simply a doubling twice (2 stops from ISO 100 to ISO 400)
Here's another way to look at it using the published guide number chart the OP posted. At M1/128 the guide number is 1.5m. If the OP changes the ISO on the camera to 400 he has essentially doubled the light sensitivity two times (100-200-400). It's as if he is now shooting the flash at M1/32 power. So read the guide number of your chart for M1/32. The chart says 3m.