welcome.
The blazer 4.3 has a very stressed design internally. Do not lean it too much. going for high teens mpg will at least keep it going. I refer to the crankshaft. All engines could be double in greater fuel mileage, oem designs prevent it. the 4.3 is a rare one to just simply leave alone, like 90% of inlines...
today smaller displacement and suffering how much cannot be done with your vehicle is about the only way to start with a strong base for good fuel mileage, and go from there.
It even keeps 3 cylinder geos popular. If there was four cyl doing the same consumption for thousands of pounds of work over it...we'd be seeking it hey?