That goal by Jone Samuelsen is now ratified as the world's longest distance goal with a header. Just five weeks after Samuelsen's goal for Odd Grenland vs Tromsų, a Japanese second division player named 植田 龍仁朗 (Ueda Ryujiro) headed in from his own half for Fagiano Okayama vs Yokohama.
Ueda-san's goal was initially claimed as a new world's record, but subsequent remeasurement of both goals using lasers established that Herr Samuelsen has the record by 33 centimeters.
The Samuelsen goal is linked above; the Ueda goal (and the piece of pretty awful goalkeeping which allowed it to happen) can be seen here. (Commentary is in Japanese.)