I have a Cannondale Beast of the East 1 with 650b+ wheels tyres. I want to buy a pair of 29 wheels which are fine to run in frame. My question is the whole drive train is off set 6mm so current wheel is dished 6mm. Would the new 29 rear wheel have to be dished?

Yes,
It’s a simple process though, tighten up one sides spokes loosen the other.

Rotate nipples by same amount. 1/2 turn each?

Repeat until rim is central. 6mm isn’t much of a shift though.

*not an expert but built a few wheels.

Great cheers.

It may work without issues would be chain/tyre clearance and or riding no hands will require a lean