The 305 78 nova I'm working on needs some more coolant. It hadnt been run in a year before I started running it. Should I flush out the coolant and replace or just add some to what it has now. How much coolant would I need to fill the whole system?
Coolant looses it's corrosion protection over time. So if it's farily old (or age unknown) it would be a good idea to drain it all out and refill with fresh coolant. To completely drain the system, you'll have to remove the two drain plugs on each side of the engine block in addition to opening the radiator drain. Those drain holes in the block often get plugged with rust & sediment so you might have to poke through that with an old screwdriver or something if nothing comes out when you unscrew the plugs.
I believe the whole system takes about 4 gallons to fill when completely empty. Plus another quart or so if your car has A/C and/or the heavy duty radiator.
I wouldn't remove the block plugs. If you want all the coolant out of it take the t-stat out and bolt the housing back on. With the upper hose off the stat housing but connected to the radiator put a garden hose inside the upper hose. Wait until clean water comes out of the t-stat housing. Turn off the water and remove the lower hose from the radiator. Put it all back togehter and mix the coolant factoring about a half gallon of clean water still in the engine.