I have seen many posts where others have done the grommet routine. I'm not sure that makes it okay. Just not sure what would result if the gromment ends up down inside the engine. If there is a grommet with a flange on the top edge to keep it out of the engine, I imagine it would be fine. But I still think the adapter is the easiest and most permanent solution, and would look factory correct. It also offers positive attachment to the engine with a bolt.
The method I devised was intended for a later engine with no draft hole in the rear of the block. There are easier solutions for a pre-68 engine. This is the thread showing the early Corvette covers on a 350 with no visible holes on top:
http://67-72chevytrucks.com/vboard/s...d.php?t=366853