We have an '88 GMC Sub with a 350 and couldn't be happier. Runs like brand new with over 226K on it. Gas milage is OK - right around 13 mpg.
However, we also have a '76 Blazer with a NEW crate 350 (less than 20k on it) that gets 7-8 mpg! I've quit driving the Blazer entirely - hoping to find a rod prospect for the engine and drivetrain (I don't care if a rod gets 8 mpg).
As Richard says, get yourself a service manual. We've got a Haynes, and it has been a lifesaver for me in the garage. I'm assuming you did all the obvious things already, so I'll just wish you good luck!
