These guys have pretty much covered everything!
I don't have near the towing weight that you do, but, your truck SHOULD be able to pull what you have. My trailer is a mere 3600#, but when I bought it, I had a Nissan Frontier (max towing weight was 5000# with an equalizing hitch, which I got). I was able to pull that thing over Monarch and Red Mountain Passes in Colorado (above 11,000 ft) but I was in 1st gear all the way. I found I was starting to run hot pulling it up a long 7% grade in Arizona south of Camp Verde and that's when I decided I really needed a bigger truck. So I got the f150.
You have a similar weight to "pull" ratio that I did with the Nissan. If you have not already gotten the hitch, I recommend the Equalizer load-leveling, equalizing, anti-sway Hitch:
http://www.equalizerhitch.com/. Others have said you don't need the anti-sway feature, but it's kind of free with the design and I'll take any advantage I can get. It's dead easy to adjust the hitch ball height and it sounds like you'll need to drop that ball quite a bit since you've lifted the truck.
The good news about backing a long trailer is that it won't want to jackknife like a shorter one (mine). The bad news is you can't see a damned thing while you're turning while backing and it's really hard to tell where the back end of the trailer is. You have to have an assistant (I think).
Get used to the stopping distance, don't overload the trailer, don't drive faster than 65 mph.
And, you MAY discover that you need to install gears with a higher ratio than you have. With larger tires, I can just about bet on it.
You WILL get single digit gas mileage! Sorry!
- Jack