Those bikes are made from High Tensile steel. As such you would be prefectly safe in rigging up some form of jig and using a jack or big lever to bend the seat tube back straight. Just don't begin wailing on it with a hammer. Use pressure to bend it, not impacts. How you'll set up to bend it back will have to be up to you and some ingenuity since it'll depend on your tools and resources. I'd say it would be worth bending it back before you rebuild it. If you're going to put that much effort into it you may as well start with a straight "foundation".

A car jack or hydraulic bottle jack could supply the force. Then it's just a case of rigging up some way to hold the frame in such a way that you can apply the force to it. Some lumber screwed or nailed together along with a tie down strap or two could be used to produce a quicky frame jig suitable for doing a job of this sort.