intake Manifold screws

i have a toyota Siena 2005 and i'm trying to remove my intake manifold so i
can get
access to my rear spark plugs so i can replace them.

i need to remove the 4 hex screws on the front of the manifold that connects
the
upper intake to the lower. My problem is that one of the bolts head broke off
while i
was unscrewing it because it was really tight. Does anyone know how
i can get the rest of the screw out so i can replace it? Does anyone know how i
should remove the other 3 bolts without them breaking off on me? Should i put
a
flame to them and heat them up? your comments are welcomed