how long do these trucks last?

[Kurt L]

i've got a 1991 cheyene that has 300,000 on a 4.3 v6 in it.
replaced the intake gaskets,alternater,water pump,regular
oil changes,tuneups.put dual exhaust on it last yr after
14 yrs with the orginal exhaust on it with catalictic
converter on it.made a world of diff with dual pipes on it
with turbo mufflers on it.got better gas milage to.
just came back from tn last weekend with it also
went over 500 miles on 3/4's of a tank best mileage i've ever
gotten with it.but dropped tailgate,didn't run ac either.
average speed was 70 miles an hour also.