I would not be very happy either with that mileage after spending a lot of money to build what sounds like a great engine for your Jeep. Yep, you're right-big tires, lots of dirty air under the vehicle, and lack of a higher gear are all contributing factors.
Let's start with the manifold first. Hopefully you bought the Performer Air Gap and not the RPM Air Gap manifold. If it's the latter, the match-up between it and the Holley Pro-Jection may not be good. In fact, the manifold may just be the problem as it was designed for a four-barrel carb and therefore you must be using some type of an adapter. You may just not be getting proper fuel distribution.
Have you done a check of each spark plug and viewed the burn pattern for a rich or lean condition between cylinders? Next is the camshaft. I doubt that the shorter pushrods are the problem, but the cam may not be lifting the valves to the full open position, and most likely the cam-to-valve-stem angle is a bit off and may just cause some sideways pressure on the valve stem, which will cause early valve guide wear. The cam you picked is pretty mild and one that I would have recommended, but-it just may be the influencing factor in at least part of the poor fuel mileage. I wanted to compare it with a stock cam but didn't have much luck, as the factory measures duration a bit different than the aftermarket cam companies do.
A couple of things that I would check before changing out the manifold (and I would suggest calling Edelbrock's technical services on this), would be that the timing is correct and that the distributor is advancing properly. This is where a good quality dial-back timing light really is a big help, as you can dial in the timing setting you would expect the distributor to be delivering at a given rpm. My guess is that you really don't want much over a maximum of 32 or so degrees of total timing at the rpm that equals the vehicle's normal highway speed.
Oh, and those IROK tires just may be part of the problem. They are a true 36 inches tall and very heavy. Combine them with a 10-inch-wide steel wheel, and the amount of weight you're trying to move is pretty staggering.
• Tech Letter Of The Month
Which Wheels to Go With?
I am restoring a '79 Ford F-250 4x4 with Dana 60s front and rear. I would like to go with 35-inch rubber on American Racing Outlaw II 16x10 wheels. My problem is that no one seems to be able to tell me if these wheels will fit. Most guys tell me to go with the 16x8s. I have found the right lug pattern, but am not sure of the offset. Please help if you can, as I'm going nuts with so many different answers.
Wheel width has a lot to do with personal choice, driving style, and what you expect the tire/wheel combination to do. There is also the clearance issue.
American Racing makes the Outlaw II in both a 16x10 and a 16x8 size, and both have the same backspacing, which is 4.5 inches. Your truck could have come with 16x6-, 16x6.75-, or 16.5x6.75-inch wheels. However, I was unable to locate the backspacing information for these. It shouldn't be too hard for you to figure out. Just lay a straight edge across the rim flange on the back side of the rim and measure down to the wheel mounting surface. My guess is that it will be somewhere around 3.75 inches, but I may be wrong.
Now as to the proper width: The narrower 8-inch wheel will cause the tire to bulge more and offer better rim protection. Depending on the sidewall design of the tire, you may experience more tire roll, especially at lower pressures, and perhaps a bit of improvement in ride quality. Most likely at higher air pressures, the narrow rim may cause the tire to "crown" somewhat and exhibit center tread wear. There may also be some tire contact due to the tire bulge with the frame under full turning lock. The wider 10-inch rim will put the center of the tire about one inch farther out from the frame, giving the truck a bit wider stance and most likely preventing frame contact on full turning lock. The ride will be slightly stiffer, as the tire's sidewalls will be straighter up and down. You may have some fender clearance issues when the tire is turned and the suspension compressed, but I don't think you will.
Bottom line: If it was my choice, I would go with the 10-inch wheel.