Autolite and Champions seem to be the most preferred for plugs, I've heard of some people having problems with Bosch, but a brand doesnt get to be that big for making crappy products (I guess).
Sludgy/dirty throttle body? Vacuum leak? That kind of MPG? Start with a plenum replacement. DodgeDude and Merc are great sources of info on this, check out DDs thread at the top for more info, then read mine (If not plenum then what) for some good info.
Then watch this walkthrough on youtube several times, it REALLY helped me out. Make sure you torque everything right, remember there are more than one components that are in inch lbs, not foot lbs, and DONT try and convert inchlb to footlb. Rent the proper tools if you dont have them.
After that, if the problem still persists, start looking into fuel delivery. The pump units in these trucks aren't known for their longevity, although they are much less common of an issue than that damn plenum.
You may also need to replace the three sensors on your throttle body, but try cleaning the TPS and IAC before you replace anything (they aren't expensive, but certainly more so than a can of carb cleaner...). Also check your PCV valve, as these can often cause similar symptoms to what you're describing.
What I would do (and wish I had done) was do the plenum job and cleaning job, establish a baseline for what the truck feels like stock, then start looking into simple/basic mods to improve MPG and driveability. Throttle body mod, exhaust, programmer (Hemifevers SCT seems to be the most liked around here), cold air intake (although there is still debate on weather or not these really help, some people have success, some don't), cap rotor plugs and wires, oil change (Like BBartow said, everyone has their preferences, although I've heard more than one person claim they saw a significant jump in MPG with Amsoil) potentially with a flush if you don't know the vehicle history and what weight/type has been run in it, and most importantly KEEP YOUR FOOT OUT OF IT! The single biggest mpg effector on a properly running vehicle that YOU have control over is your foot. Be gentile, and you
could see upwards of 18mpg (some claim more, I claim shenanigans).
Also, don't expect it to get the mpgs of a prius or subi, or even newer trucks. The original 318 was designed when gas was pretty cheap, so keep in mind that it was never intended to be a gas friendly engine.
Hope that helps get you started in the right direction, this place is full of very very knowledgeable people. Just remember that everyone will have their own opinion and own way of doing things, so take it all in stride.
. And I pass like an *******.
