Were those 50 year old spark plugs in aluminum heads?  Heat should only be
used with extreme caution when trying to remove any bolt from aluminum, it
doesn't glow and then melt like iron, giving you a gentle warning.  I didn't
read the original post, but would advise against heat in removing anything
from a head.  That's my $0.04 worth.  (I am a mechanic, I charge more)
Patrick