I remember that picture,
it convinced me to repack my Thunder exhausts as well

my set, without a protruding internal tube, have only 1 setting screw at the rear bottom.
the only place where the inner tube touches the outer tube, is at the rear cone.
I gently hammered the outside pipe down onto a soft wooden block, and the inside came out with 10 taps.

you could try to gently tap the rear cone edge inward, just to break it loose.
if you need to add heat, the only place you need to heat, is the rear where the inside cone touches.
can you at least wiggle the inner tube a bit, or is it completely stuck ?
if no movement at all is possible (left-right / up-down / in-out) are you sure all setting screws are out ?
you could also try to tap the inner tube out from the rear, with a rod through the connection holes
for the header pipes, after removing the headers pipes, (they wiggle right out)
but first mark each connection with a alcohol marker, and/or take pictures of which tube goes where, from both Left AND right side! so that you know at which position to put back what pipe
making the packing wet is not the way to go, they will rot (but that is not your concern when trying to replace)
Anthony