Personally, Id hunt for a 350. It has more power on demand, no physical difference exterior wise, and can be made to perform better with simple upgrades. That being said, my 86 extended cab gets about 19-20 hwy with the 305 /700r4 combo unless its windy. then it get down to about 15. Keep in mind its a 2wd tho. I really am not sure what mileage I get out of my 85 k10 with the 350 because due to rear axle issues, I havent had a chance to really find out. I would definately ditch the 2 barrel when possible and upgrade to a 4 barrel, with a rv cam. This will help you both on and off the trail. The 6 is a better trail engine in my opinion because they are notorious for low end torque. Yes the 305 has more, but it will be at a higher engine speed, and may result in more wheel spin than beneficial.
As far as the cold air induction, for years I drove Oldyellers 81 C20 with 350/400/4:11s with the air cleaner lid flipped. It was what everyone was doing for supposedly more horsepower. The I had an old man tell me to put it back on right, and hook the tube back up. With just that, I went from 9 mpg to about 11mpg. It seemed to run better too. Later, he rebuilt the carb for me and I got almost 15mpg. I was very happy, plus it ran like a striped ape. If I can get close to 15 with a 6000 lb 3/4 ton truck, your jeep should be fairly simple to get there. Main thing is a properly tuned engine, with a properly setup drivetrain. On your Jeep with 31's, I wouoldnt go more that 3:73s and that jsut because it seems the next upgrade is always larger tires. That engine with 3:73 should turn 33s pretty easy and not hurt your mileage too bad. Most economy is ruined because the final drive ration puts the engine in a lug condition under normal driving situatuons.