Looking at a 95 K-2500 w/ a 454. It has 157,000 miles and is around $5,500. It is pretty clean and no leaks. My question is, how does these do on gas mileage? We were hoping for a 350 but these trucks rarely come up for sale where I live. Is there a way to tweak the computer for better MPG? Also if we wanted to, is it hard to swap engines with a 350 later on?