Congrats on the truck. You already got the air intake. Grab a diablosport predator and throw on an aftermarket exhaust then see where ya want to go from there.
The electric fan and 180* t stat are reasonable upgrades that you can do in your own drive way. Also check into fastman
if you are interested in upgrading your throttle body. If you have some downtime for like 3 days, ship your TB in to be ported/bored and this will save you a lot of money instead of buying a brand new one.
From my research on the forum I hear that the two most popular mufflers are magnaflow and flowmasters. I believe people see more performance out of the magnaflow as compared to the flowmasters.
There are a lot of different options when it comes to headers. If you do decide to go the route of aftermarket headers do your research and ask around the forum. I just got my pacesetters in. I bought them raw (cheaper) and am having a 2000* ceramic coating put on by a local shop. I'll post a thread about the gains when I get them on the truck.
As for the intake manifolds and cams, I'm not the guy to ask. I believe there is a little more work that has to be done than just plopping a 6.1L intake on your truck, same goes for a cam obviously. Plently of experts here on the forum that can help ya with that though.
Another mod you could consider is gears: 4.10s or 4.56s. Just know that these would have a negative effect on fuel milage. Although I hear the 4.56s turn these hemi's into beasts.
As far as fuel economy is concerned when modding your truck the only thing I have to say is: it all depends how you drive it. If you drive responsibly you will see an increase in fuel economy (also diablo offers a MPG booster which helps a ton as well). If you hammer it every time you leave a red light... well your not gonna see a pretty number for your MPGs.
Congrats again on the new truck!