take the head off and take it to a shop
if you've crossed the threads at some point it might not be as clean a job to remove it and you might have got some shit down into the bores

we had a 300 td merc in a few months ago and the glow plug striped it's threads on the way out
it totaly fucked the piston as bits if the thread mashed themselves into the crown and the chamber where the valves sat was all fucked as well

with a bit of heat everything came apart and all it needed was a new piston, but there was blow torching to buggery going on with that head to heat it up enough to get the plug out and then we had to get it checked to see if it had warped or not