I bought a 94 F150 5.0 xlt 4x4 a few months ago. It sat for a year with. Guy thought it was a blown head gasket. It was just a freeze plug. Another one just cracked last month. Should I be concered about the others? It was running with just water in it. I've flushed the cooling system replaced water pump and thermostat. Was fairly rusted throughout the cooling system. Thanks for any inputs.

 

This is the first after I replaced it. Was cracked on the edge. The other just had a hole in it.

 

Yes, if two plugs have failed the rest are probably going to fail soon. I would replace the ones that you can. Always use coolant and distilled water or premixed coolant to prevent corrosion. Tap or well water can have a lot of minerals and they will eventually clog the system. Some oldtimers used to use rain water.
Also make sure the coolant recovery system is working to prevent air from entering.
regards
rikard