Shouldn't have to remove the whole dash. I just did mine in my 00 GT. Had to remove the entire center console section (where the shifter and cubby reside) Pull the cubby organizer out and you will see some nuts/screws there. There is also 2 near the ashtray up front. IIRC, the console top just snaps in place so be carefull pulling it up as to not break any clips.
Once the console is out, move both seats all the way back for room. Take a look under your dash dead center and you will see a the black air box (the part that directs air to the floor). Once you remove this the core is right there. There are a few bolts holding it in place inside the box that need to be removed, and of course don't forget to remove the heater hoses from under the hood.
I'm used to doing cores in Ford vehicles (where you do have to pull the entire dash) and had NEVER touched a GP core before and I got mine out, back in and running in about 2 hours, without the help of a Chiltons.
Search for heater core replacement here on the forums and I'm sure a guide will come up on how to get the heater box from around the core. If you can't find anything useful let me know as I may have a guide from when I was looking into it.
