The battery should be grounded to the block. The starter needs the most juice to operate, and by grounding the battery to the block you are making sure that best path is maintained.
I bought an old GMC truck a few years back for $50. The seller stated it wouldn't start, even with a new battery. My brother and I noticed that the battey was grounded to the body, and not the block. We paid the $50, switched the ground wire in the sellers driveway and she fired right up. You should have seen the look on his face. And OBTW, never had another problem with getting the starter to turn over.