The heater plemum is fastened to the body with 6 large, #3, phillips
screws as David Beirl has pointed out.  They have oftem been turned down
pretty tightly, and in the salt belt the threads on the screws may have
rusted some making them quite hard to remove.  I squirt some penetrating
oil on the exposed screws.  The upper passenger side screws are
acessible from the front of the van.  Those on the driver's side are
not.  Having done that, I go after the screws with an impact screwdriver
with an 18" long 1/2 " extension inserted between the working end and
the end which you hammer.  I use a rather heavy hammer to do this, and
it almost always works to loosen the screw which can then usually be
turned out easily enough. Sometimes the head of the screw does strip
out.  When that happens I have a long, long, long, well maybe 15" long,
three eights inch drill bit that I use to drill off the head, which
releases plenum in that area.  It seems to me that the two upper screws
on the driver's side are the ones that are seized the most often.

Put some anti-seize on the threads when you install it again.

An impact screwdriver is a handy tool, one I think most of us should
have.

Chuck Hill