This has never failed for me.

Take a 17mm cresent end wrench (I think, it's about that size).
Insert the open end between the head and the tab on the
cylinder like the tab is a nut/bolt (thus, the tab is
inserted into the open end) with the larger side of the
wrench toward the head - this should make the wrench lean
toward the crankshaft. Pull very hard. Move to the other
cylinder, repeat on all exposed tabs. Use piece of pipe
if necessary (i've never had to)... The heads have popped
off after a couple times around even after I couldn't
separate the heads from the jugs with a wedge and hammer -
I think the symmetry of pressure is necessary...

dave