Just in case someone else is in the same boat, this might help.
The old Duotherm 65810-002 furnace
was shot. Gas control valve and pilot light
Removed grill and valve assy. Installed blanking plug in propane
line. Removed mounting screws (4).
Center exhaust tube was totally seized and would not release after tapping, twisting, pounding, etc.
Following a suggestion in an old thread, I drilled out the pop rivets on the vent cover, then used Dremel cutting disc to expose and cut the center exhaust tube. Furnace
slid out on the inside slick as could be. Now to close the hole and remount the vent cover.
This may work for any furnace
that doesn't need to be salvaged.