Welcome to the site. This is an old post I saved from another site regarding the King Seeley gauges. The member bergmanj doesn't post much anymore (I think he retired and only has a dial up connection) but it contains some interesting info on calibrating the gauges.
The other item I'd add, is check all the grounds. Time and corrosion often cause a problem on that side. Not just the gas gauge, but taillights etc.
http://forums.y-blocksforever.com/Topic22135-1.aspx