Not sure if this will help... but once you have drawn the 2D ojbect and before you have started to anything 3D to it 'copy' now start to extrude etc. Once you have finished creating the 3D object 'paste in place' the 2d drawing.
Now go 'Organise'>'Create symbol' a dialogue box will open, choose the setting (pretty straight forward) click 'OK'
Now you are able to look at the 2D drawing when in 'Plan" and the 3D model when viewing in 3D.
The advantage, this way is being able to dimension as you orginally wanted.
Another way would be to double click the 2D symbol once created then dimension. This way the dimensions would dissappear when in 3D.
Just a thought. hope it helps.
Cheers.
