Safest way to clean head gasket material from the block? I dont want to hurt the pistons or cylinder walls.

 

A scraper while using a shop vac will work. Just don't use any abrasives like sandpaper or Scotchbrite pads. I usually use simple and cheap alcohol to clean off any final residue.

 

how about sandpaper on top of the block and not in the cylinder walls?

 

Save the sand paper for "just in case you gouge something". I've always used good new razor blades and a small pair of Vise-Grips for a handle.

 

Sandpaper and Scotchbrite make particles too fine to get cleaned up. Stay away from them when there is any chance the particles can get in the engine. Even GM has a bulletin out about not using Scotchbrite on engine stuff. The small particles find their way to the bearings.