First of all, I would like to apologize for posting to the 4x4 group,
when I only have a 4x2.  Houston Road Runner doesn't have just a Chevy
Trucks group.  I hope you don't hold that against me.  

I have a 1989 C1500 4x2 with the 4.3 v6 and 5 speed standard
transmission.  I bought it in 1992 with only 60,000 miles on it.

the engine internal components are all original, the clutch is
original and the transmission is all original.  the truck now has
about 300,000 miles on it.  that's a guess, my odometers work only
part of the time...but that's another email.

like I said the internals of the engine are all original, the only
things I have replace are the starters, AIR pump, water pump, fuel
pump, alternator, filters and fluids.  it's been a really low
maintenance truck.

as far as I can tell, there is no oil in the water and no water in the
oil.  it's never ran hot and never been dogged, I bought the truck
attempting to not have another car note for a really long time, I know
it's a pipe dream, but I'm so tight I squeak.  the body is in good
shape, it needs a paint job and very little body work (a crack in the
fiberglass fender) and there is no rust.

Recently, I had to change the catalytic converter, because my other
one plugged up and blew off, literally, it was a factory original slip
on.  

After a week of service, the new one is about ready to do the same.
I'm sure I have oil blowing by, I see the puff of white smoke every
time I start it after it sits a couple of hours.  I'm sure the engine
is burning rich as well.  I need a new MAP sensor, but I haven't
gotten around to it.  

I would like some opinions and information as to whether or not it is
worth rebuilding the engine.  or if I should get a rebuilt/crate
engine.  what are the pro's and con's of both?  

I don't have the space to do it myself, I would love to.  if I can get
it rebuilt, where do I start?  what questions should I ask?  what are
your opinions for quality components to get me another 300 - 500k?

I'd like to keep the original block, but with 300k on it what
potential long term issues, if any, am I looking at?

if you need more information, please feel free ask, I'll let you know
whatever I can.