For future reference:

"Gilligans Island cast is named" - Reality TV World Message Board Forums - "Gilligans Island cast is named": "'The Lost Gilligan's Cast'
In case it ever comes up in Trivial Pursuit, imagine the show with the following:
Jerry Van Dyke as Willy Gilligan (turned down the part)

Carroll O'Conner as Skipper Jonas Grumby (tested for the part)

Dabney Coleman as Professor Roy Hinkey (tested for the part)

Jayne Mansfield as Ginger 'Bunny' Grant (turned down the part)

Racquel Welch as Mary Ann Summers (auditioned for the part)

Eunice 'Lovey' Howell was that character's full name.

After the series started airing they decided Gilligan was the character's first name, his surname was never revealed."