/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