heck 20 years use would be great for me,these older motors running the lower gears ,with transmitions with only three speeds didnt usualy see any were near the miles the new cars see , 60 000 without a rebuild was good ,my 2001 f150 has 200500 miles on it now and runs like a top i used to buy mustangs with only 40000
miles and the motors were completly worn out .just a head gasket in 20 years ,i would be happy with that ,double check the torque on the bolts when you replace them and go for a nother 20 ,you have to rember the older motors had higher compression as well wich takes a toll on parts as well