I have done a few of them, I dont think there is any possible way to do it without removing the dash..
once you take the dash out, it is pretty easy. you undo the coolat lines under the hood , REMOVE THE NUT THAT BOLTS IT THROUGH THE FIREWALL UNDER THE HOOD, I have seen people forget that nut & destroy the heater core housing when removing it. there are a few nuts & bolts that hold it to the fire wall on the inside that need to be removed, the cable to the water valve needs to be disconected, REMOVE THE WHOLE BLACK HEATER BOX (the actual core is inside that box). split apart that box & replace the core.
reverse this procedure to put it back together.
there may be a few little self explanitory things that I didnt write, but I would deffinately get the factory service (helms) manual if you are going to attempt anything like this on your own for the first time, the haynes manuals are no where near as good and I have found bad info in them already, but they are cheaper. you can get a factory service (HELMS) manual, same ones we use at the honda dealership from
www.helminc.com, theese things are worth there weight in gold when doing a repair or swap yourself.
hope this helps you out some
Dave E