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I presume the tank you reference is the port side saddle tank.

It is clear in the builder's diagrams that the tank sender is installed prior to the cap installation.

See the tank drawing way way down. There is a port and starboard view.

http://whalerparts.com/Diagrams/2005/...df#page=79

I am guessing you have a inspection port in the sidewall to access the fill/sender area.