Best thing should be, capture the off-line in dv via the dv board of the J3, do the off-line, than recapture in Hd with the J3. This what I do for all the HD feature I do, and it work perfectly. (I work with the same decklink HD card as you mention).
Ciao !
FM
Francois de Lorimier Milette
Communications de Lorimier inc.
Montreal (Quebec) Canada