Yes, the flush agent above is the correct thing to do. The only thing I can add is that once you have used the flush, you can rinse everything away very thoroughly on these cars with a garden hose.
Begin, by removing the hose at the drivers side rear of the cylinder head and the hose from the auxilliary water pump. Rig up a way to connect your garden hose, or if you don't mind the possibility of getting a little wet, simply hold the garden hose to the hose that you removed from the head. This will reverse flush the heater core. Flush it thoroughly until good clear water is coming out and all flush agent is removed.
Then at the passenger side rear of the engine behind the exhaust manifold is a drain port that will accept a piece of hose. Open it, and open the bottom drain of the radiator at the passenger side front of the radiator. Make an adapter for your garden hose that will fit over the block drain. With the engine off and the radiator drain open, apply garden hose pressure here for a little while to flush everything out.
Do everything you can to flush alot of water through. Remove your hose and drain out as much of the water as possible. You can blow into the heater core a little to flush out most of the water that is in there.
With everything back together, for 50-50 antifreeze/water, pour in about 4.5 quarts of antifreeze, then top off with water. Run the engine with the pressure tank cap off and let the level drop after the thermostat opens. Top it off with water and you should be all set.