Well, I managed to get the cover off and the impeller out. I did it by hiring Plastic Man to contort his arm and hand to squeeze between the cover and oil filter.
Actually, I found a very small box end wrench (6mm) which barely fit over the hex head nuts. They were fairly loose...testament to the difficulty others have had, maybe?
Managed to wiggle the impeller out using needle-nose pliers. Of course, it looked brand new.
I am definitely installing a Speedseal cover, and possibly that oil filter extension...cause I just don't want to contemplate having to do that in any kind of sea...sheesh.
Thanks to all who chimed in...
It's not whether you're paranoid...it's whether you're paranoid enough.
1988 Sabre 30