Re: He doesn't need to sterilize his water, he needs to recommission the system.
I own the same vintage Hunter as yours and had to remove the original aluminum tank due to pin hole leaks.
My yard guru had a simple solution to remove the old and install the new tank:
Cut out the settee top with enough 'lip' left over to mount teak strip braces on the inside of the opening. Then, re-install the cut out portion of the settee onto the strips with screws and epoxy the seam, with gelcoat for appearance.
It's a strong fix and allows for a simple way to access the tank in the future.
By the way, I bought the new Ronco tank from SBO, as they provide custom fitting locations (inlet, outlet and vent) based on my plumbing needs.
Let me know if you'd like me to call with details.
G.