Usually, applying heat helps loosen stuck items, however, I'd caution against using too much here because of the risk of cracking the head. It has hollow passages for water circulation and the relatively thin casting could crack. Your best bet is to continue with plenty of penetrant and pressure. Slow and steady will win the race!
If it does come down to it, you may have to remove the head if the plugs just won't budge. Fortunately, the HJ engine uses nuts to hold the head in place and the risk of snapping off a stud is very minor. Just use lots of lube on each one along with a wire brush to dress the threads.
