I would start at the bottom bracket first, since there is significant rust at the base of the frame and if you can't remove the bb cups, well don't bother doing anything else.
Spritz PB Blaster or the rust penetrant of your choice to soak into the threads. Reapply daily for 2-3 days. Then give it a try, use a torch if you need to too.
If PB + patience + heat doesn't work, you may need to dip the frame in an Oxalic acid solution, basically Savogran wood bleach, about $9 for 8 oz of powder soluble with water.
The fork can be soaked too in OA, or hit it with a wire brush and poke at it with a pick.