Believe it or not, what oil you use can also make a big difference, on such a small motor. Unless your motor has significant leakage, blowby, or burns oil, I would make sure you use synthetic 5W30,or even 5W20. If you are currently using something "thicker", you will notice an improvement in fuel economy, and you may even feel a slight bit of seat-of-the-pants power improvement.
Also, as someone suggested, a Cold Air Intake setup will also make a noticable improvement. The pre-made/engineered ones, like a K&N FIPK, are the best, but you can make your own that will come pretty close, and be a lot cheaper.
Buy a universal cone shaped, cotton filter of your choice (K&N,Volant, AEM,etc...), then build the tubing with PVC schedule 40 pipe, and rubber couplers from Home Depot or Lowes. Be sure that the filter is able to grab cool outside air as much as possible, and is not sucking in hot under-hood air. Also, for added heat-rejection, wrap the pipe in self-adhesive insulated foam, with the aluminum coating. It's also sold at Home Depot, as pipe wrap, or hot water heater wrap. It's fairly thin, and comes in a roll.
Also, in addition to the hi-flow cat, you can also install an aftermarket, or custom bent exhaust, with SLIGHTLY larger than stock pipes/better flowing muffler/etc... Don't go too big on the pipes though. That little motor needs all the torque it can get.
As for MPG, there a few little things you could do, that might help a bit.
1.) Wheels. If you currently have steel wheels, consider picking up a set of junkyard stcok aluminum wheels. For someting this old, you should be able to find a set of wheels fairly cheap. Remember, a lot of newer wheels will also fit, just watch the center hole size, if you have 4x4, as the hub needs to pass thru. (Jeep wheels, unfortunately, won't fit because of that, even though the bolt pattern is the same.) Don't forget the spare too.
You could easily save 10lbs per wheel. Not only will the static weight reduction help, but wheels are rotating weight, therefor this the most significant place to save weight. The truck will accelerate and break slightly quicker, and may even handle a bit better.
2.) Overall weight. Clean out any crap you don't need to carry. This includes the cab and the bed.
3.) Aerodynamics. If it's a 2wd, you could lower it little, fairly easily. That would reduce drag, at highway speeds. If it's a 4x4, then forget it. Given what the truck is, and your stated goals, it just wouldn't be worth effort.
In addition, you might try building yourself a front lower air dam, to keep some of the air form getting under the truck, at highway speeds.
A bed cover will also help. If you don't want to buy one, you could probably make something. Just be carefull with the materials and construction. It would be counter productive to add a whole bunch of weight.
www.ecomodder.com
http://aerocaps.com/
http://i.autobloggreen.com/2010/01/...cs-truck-owners-get-in-on-the-aero-ecomods-t/
http://ecomodder.com/forum/showthread.php/list-aero-mods-you-can-do-your-vehicle-83.html
If you're really feeling ambitious, you could try to build a complete undertray, to close up/smooth out the underside.
Also, as someone else mentioned, and electric fan, to replace the stock mechanical one,would be a noticeable imporvement in economy, and power.
If there underdrive pullies available for your engine, You should definitely get it/them. By reducing the speed of your engine driven accessories (P/S, a/c, alternator, water pump), it reduces the amount of engine power needed to drive them. On such a small engine, this can amount to a significant percentage of the engine's total power.