Something like this:
http://www.harborfreight.com/1000-wa...ght-66439.html
I have one similar to this, and it has a high and low setting on each lamp. They get pretty warm, so they should be able to keep your painted area at a decent temp, assuming it's not like 20 below in your shop. If you can keep your shop in the 40s or 50s, it would make painting a much simpler task. If not, paint a small area that the light can keep warm, them move on from there. It will take longer, but it beats waiting until it's warm enough to paint without it.
I would definitely try to heat the metal up to about 65 degrees or higher first. That way the paint that's touching the metal won't be soft for weeks while the top cures. If you can keep the metal and the top painted surface at a similar temp (say within 10 degrees of each other), it will cure pretty evenly all the way through.
It sounds harder than it really is. If you've got a small space heater, use it to heat up the frame, then move it while you paint. After you're done painting, put the heater behind the frame, but not so close to cause the paint to heat up too high, and put the shop light on the other side of the frame so the surfaces can maintain an even temp.