Your fuel pump is most likely the cause.
gettin to much gas to the carb
inside the gas tank
87 octane should run just fine.
My stock 350 is 14city 19 highway.
NO, gas parts and diesel parts do not need to mix.
The actual mileage that can be achieved in a Chevy depends on which Chevy you actually own. For example a well tuned Chevy 350 will do around 14 miles per gallon.