[nUcLeArEnVoY]

You don't have to pull the entire dash, no. You do need to get the glove box out, and the more ducting you remove, the easier it is to pull it out and stuff it back in. It helps to remove the passenger side vertical dash vent (not the chrome ball chiller one), and that duct is challenging to reinstall because one of the screws is very difficult to get to with the dash still in place. Re-installing it is significantly harder than pulling it out, since the clearance is minimal thanks to the heater core outlet tube, and obviously you're going to want to be careful with the new one since the joint mating the tube to the matrix is very fragile.

And while it's not mandatory, it makes the job a lot easier if you pull the fender well, which is also challenging to reinstall since it's sheet metal rather than plastic like modern cars. While it's out, you may as well replace the blower motor if you haven't already.

Finally, to help prevent this issue in the future and extend the life of your new core, either replace the heater hose fitting in the passenger cylinder head if it's a Pontiac engine, or maybe place an in-line piece of bar stock with a 1/4" restriction in the inlet hose and clamp it down with worm gear clamps so it doesn't move. The fitting in the cylinder head has a similarly restricted rubber insert in it that usually disintegrates over 40+ years, as was the case with mine. The reason the factory did this restriction was to prevent water hammer damage to the heater core during sudden acceleration that causes a sudden surge in pump pressure. Over time, this stresses the heater core and can lead to early failure.