Its kind of expensive but a good set of headers with a 3" pipe and free flow'n muffler will help alot, mostly the headers. I think theres only one company that makes the headers and their top end stuff. Its been awhile since i've owned a EFI 460 but it seems like they were about $600 to $700.
You're thinking about Gale Banks. The headers are closer to a grand, but the engineering and build quality is second to none.
As for better economy...
First, a good tune-up. Plugs, wires, cap, rotor... you know the drill. Change the fuel and air filters, run some injector cleaner through it (probably several tank's worth), and generally make sure all the basics are done right. A switch to all synthetic lubes - engine, tranny, rear diff - will help a little too, and you can count it as the preventive maintenance it probably needs anyway.
The most cost-effective long-term option is probably a taller rear gear. You could spend a grand or better on other options that might net you a mpg or two, or $600-800 on a having a gearset installed that would definitely get you that kind of increase. Of course, the trade-off is dealing with lackluster acceleration and pulling power.
In all honesty you could probably sell the truck and purchase another truck with a V6 or small V8 for at or near the same price that would haul firewood just fine. That might be a more sensible option if the that added economy is that big a thing for you.
Keep in mind, though, that plowing a bunch of money into economizing a vehicle only driven a few times a year gets kinda tough. If gas is $3.25 a gallon and you eke out an additional 2mpg over your 10 mpg baseline, that's a savings of 5.42 cents per mile. To recoup $1000 in mods you'll have to drive the rig a minimum
of 18,450 miles. If you only put a couple thousand miles a year on it then the expense may not be justified. Ultimately it might be better just to tune it up, perform preventive maintenance, and deal with the crappy economy the few times a year you use it.