Nice thing: early GM vehicles (the plain ones, at least) had fairly simple dashboards and therefore the HVAC components aren't buried. I've replaced heater cores on a Fiero, an SL2, and for you Ford guys, my '88 F150. The F150 was easiest: drop the glove box, pull the hoses off under the hood, 4-6 screws to remove a cover, and the core slid right out.
That was back when things were simple. I'd be willing to bet that 90%+ of vehicles made in the past 20 years require at least some amount of instrument panel removal for access, easily a 6-10 hour task.