Are the plugs rusted in? If so, try some Aerokroil. Fairly amazing stuff for freeing up frozen fasteners...mine came right out after a spritz and overnight soak.
Guess you can flush on a best-effort basis if it's absolutely impossible to get those plugs out, but they allow the water jackets to almost completely drain so you really want 'em out! Assume you know to do whatever you can to remove as much of the water as possible if it wasn't distilled and to flush the heater core after to clear possible contaminants. Then refill with a good heavy-duty coolant/distilled water/SCA solution, or take the opportunity to install a coolant filter with SCA release tablet.
Might want to have a look at the thermostat and the small t-stat housing check ball while you're doing the radiator...my check ball was gone when I looked. On my 88, I wound up replacing pretty much all of the cooling system components since I only wanted to do that teardown once - W-pump, t-stat, housing, fan clutch, belts and hoses, along with the rad.
Jim
Can I just fill and drain a few times? It did have good sca's in it but the rad got a hole in it and I had to limp it home with water. Any help would be awsome! 
