Re: Removing prop from 270
Yes, it does turn counter-clockwise (just like a "normal" nut) to take it off.
If you have managed to get the tabs removed from the grooves, it should come off. Ensure that you do not have a "bastard" with a locking bolt (allen key 1/4" needed) in the center of the hub as well.
Have you sprayed it with some penetrating lubricant and waited? The prop cone is alloy with (stainless?) steel thread inserts. It is possibly that the cone has been on for so long that it has corroded on the inside.
Volvo prop cones are a "dime-a-dozen" and can be had for $20-$40 in case you want to use so much force that you damage it beyond repair.