My GM Manual just has pics.  It shows 12 nuts/screws to be removed.
However the astro/safari forum has much more at:
http://www.astrosafari.net/
You can search without joining.

Let me quote a bit.  Beats me if any of this will help.
Quote 1:
Am trying to remove the oil pan on my 1996 Astro 4.3L motor and
it seems to be stuck. I have the timing chain cover, starter motor,
and oil filter housing off. I have the 10 oil pan bolts removed and
the 3 bolts in the rear of pan that bolt to the bell housing off also.
The crankshaft key-way is at the 6 o'clock position straight down.
With a 4 x 4, I have struck, and struck hard at the lower sump
area and can't break it loose. I can pry on the front of the pan
and see a little movement, but can't get it to come off. Is there
something I'm doing wrong or something I've missed? It seems
to be stuck at the rear just before the last bolt. I see sealant
has oozed out of the seam in this area (factory seal). Could
that be the problem? A sealant so good that it's more
like a weld??

Quote 2:
I have a later Haynes manual & this is the only differences
I can see between what it says & your description....

Remove bellhousing lower cover completely.
It says timing mark on damper at 6 oclock. Should rotate it
clockwise to this position using damper bolt.
remove oil filter adapter
remove rubber bellhousing plugs
other than this it says whack it with block of wood & mallet,
rear portion comes down first. Don't pry it with a screwdriver,
you'll never seal it again (unles you can knock it back into shape).

Quote 3:

I belive you need to remove those two rubber plugs from the
tranny bell housing.  There are two bolts *way* up in
there that need to come out.

You might try specifying which model you have and a few details.
...thehick

Did you remove the two nuts that are "inside" the bell housing?  You have
to remove the two rubber bell housing plugs and use a deep socket and
extension to reach them.

Ian