/6 rebuild advice
if it has stood for a long time, id expect all kinds of crap in the coolant
i'd also expect mayonnaise in the oil filler cap caused by condensation in the block dripping in the oil, for years.....
dump both oil and coolant flush cooling system
whack in some K seal and plain water for 100 miles nice hot miles
dump that do not leave it sitting in there
Replace with proper coolant
enjoy, anything needing doing will now be apparent over your known good baseline
Dave