I've done a lot of mechanical work, and the heater core is probably one item I would almost pay someone else to do. Realistically, first time out, it's a 6 hour job, but doable. If you bypass the hoses, make sure you run your U-hose on the hard heater lines, dont loop the dead core together instead (how it sounded to me). You could just not use the heater/defront until you swap it, it doesn't circulate through there unless it's on.
Brandon