I believe the general rule is to use the earliest recorded version, in it's native spelling.
Harald is the one I see in most sources; I also point out that the Danish sources (Danish wikipedia entry) indicate Klak (only one a) is a nickname, possibly derogatory and given to him after his death. His patronymic(?) sees to be given as Halfdannson.
I also believe it should be Jarl not the anglicized Earl.
http://www.vikingekonger.dk/Vikingekonger HTML/Artikler A/Harald Klak og hans familie.htm