Try avoiding hacking the post and, the other stuff. try removing seat and seat-securing assembly and get it in a vice, pinch up but don't crush it, then, wiggle the frame with increasing movement. it just may be enough to budge it. the post is at an advantage at that height already. IT's when you have over 7" in there that you get worried.
If it still wont have it, freeze it. either get a kit that plumbers use to freeze riser mains etc... or, buy some 'crack-it' from ebay and manufacture your own insulating jacket from foam/sponge and a carrier bag. this method does work and has virtually no bad consequences to paint etc...
nice bike by the way.
Ride yorkshire on an old bike.