Soak the spline with a penetrating lubricant for a couple of days first. Give it a good healty drench in the morning and at night.

Once you've given it some time, use a piece of scrap lumber, natural (structural lumber), not pressed board, MDF or OSB. Grab yourself a sledge hammer and smack the poo out of that sucker.

Although, if you've already started to mushroom the head, you may have a LOT more trouble in store for yourself.

Not to mention that even if you do get it out, a wise auto shop won't take it as a core exchange if the threads are messed up.

The key here is LOTS and LOTS of penetrating lubricant... and patience.