Removing and brazing in a new seat tube should be your last choice, even though, contrary to what two other CR listers wrote, this frame repair is not terribly difficult (you cut out the seat tube, file and ream it out of the bottom bracket and seat lug,and braze in a new one).
Ream out the broken seat post (like Don Walker wrote) before using the hacksaw. IIRC, Jobst Brandt wote about this on the rec.bicycles.tech list. I've also read somewhere that the Coke trick works.
Good luck!
Dan Christopherson
Lopez Island WA