I also forget the most important thing to know about a 12 valve Cummins; google "killer dowel pin" for plenty of information on what will destroy a Cummins. Don't let it frighten you, I have repaired this on two trucks and it's not that big of a deal. It looks harder and scarier to fix than it really is.
You can search down as rear axle with a 3.08 ratio and swap it in and pick-up maybe 1 or 2 mpg I believe. They were also available with a 5 speed Gettrag trans.