Sheldon Brown had an article on stuck seat posts, some of which is applicable to a stuck stem: http://www.sheldonbrown.com/stuck-seatposts.html He recommends penetrating oil for steel, but ammonia for aluminum (which I assume your stem is made of?). Hel also recommends heating or cooling. For steel, heating the outside tube is recommended. For aluminum, cooling the inside tube is recommended.
This method has allowed me to get out all of the stuck aluminum stems I have encountered:
1 - The ammonia trick seems to work, usually I flip the bike upside down and pour some through the bottom of the fork steerer and let it sit for a while.
2 - Put an old wheel in the fork and clamp a 25.4mm (1") bar in the stem. If you have an old set of weights the handle out of that should be heavy steel, long, and 1" in diameter. If you don't have that then an old steel mountain bike handlebar could work. Assuming whatever you clamp in the stem is long enough it will give a lot of leverage and you won't have to worry about messing up the wheel.
3 - Give the stem a couple taps down with a hammer, use a piece of wood to avoid damage.
4 - With the bike right side up put the wheel between your legs and grab the bar. Twist back and forth trying to break it free. Usually once it moves a little you should be able to keep going back and forth while pulling up a bit at the same time eventually working it out.
If it still won't come free try putting some liquid wrench or wd-40 in through the steerer bottom, let it sit upside down, then try again.
I've had success with the following: Unscrew the stem bolt until it's protruding. Squirt some WD-40 or similar penetrating liquid in there. Give the stem bolt a couple of crisp, gentle whacks with a claw or ball peen hammer. No wood or anything to soften the blow. Done.
If it's a 1972 bike with original equipment, I'd be surprised if it isn't a steel/steel interface. Sometimes rust and dried grease get in the way even if no aluminum is involved.