[oil pan 4]

If you are going to add a big front bumper you might just want to stick with an air dam, the belly pans are a lot of work since you are going to change the bumper anyway.
I think the idea with belly pans is to keep them as low to the ground and as parrel with the ground as possible. It looks like I am going to build a custom bumper to house the intercooler for my diesel so I will kind of be doing the air dam thing too.
Depending on what kind of towing you do you may not want electric fans. The duramaxes went to electric clutch in the early 2000s. That may be your best bet for cooling.
The problem with thermal clutchs is they dont seem to ever really fully disengauge. I picked up 2.2mpg when I did my belt to electric fan swap back in 2006.

I may add air bags too. All ready have compressed air on the truck so might as well.

That transmission isn't going to die unless you really try to kill it.

You may want to figure out what gears you have in the differentials and think about swapping them if you really dont need them.
I use to get by with 3.08 gears towing 6,000lb with no hills.
Since I haven't towed any where near that much since 2007 I am thinking about building up a rearend with 2.73 gears now that I have it turboed.