I'd go with what's rated since your mods might make your truck go faster, but they aren't going to make it stop faster.

Here are some docs that will help:

 

I also have an 03, read my signature. Our gvwr is 8800 pounds, or 20,000 for gcwr, so per Ford this is there rating. Now, I am tagged legally at 24,000 pounds so if for any reason I get pulled over I am covered. I also travel for work as well as play and needed a 5er this year. Mine is 42' long, weighs over 13,000 dry, my gvwr is 10,000+, gcwr is just under 23,000. I have no issues pulling or stopping this weight, but I also have a Class A license and have experience heavy hauling and driving semis. I could feel my 10 ply tires squirm a little and I was close to the load rating on them so I went with 19.5 wheels and 14 ply g rated tires. This made a tremendous difference in how it feels at highway speeds. Biggest issue like mentioned above is stopping for people, not sure why but some never check or adjust there trailer brakes as required. I am sure others will say how especially with the new Dexter axles on the campers the brakes are self adjusting. Well yes they are in a perfect world but reality since most don't pull there trailer often those parts seize and don't operate right. My new 2017 had to be adjusted manually, since doing there recommended break in did not work. Also do yourself a favor and get an Anderson ultimate hitch if you do buy a 5er. It is light, easy to remove, easy to install and hook up, and pulls awesome.

 

Here is my empty or dry weight just truck, trailer, and myself.

 

I have a 34' fifth wheel that is rated at 11k fully loaded. I could go heavier by the owners manual, but with all the different weights like hitch weight and axle weights, I felt comfortable going to 80%. The truck is a 2005 Ford F250. After I got it, the truck sagged, so I added air bags. To go to the max weight, I'd feel better getting DRW.

Other thing I noticed is that you mentioned towing with a gooseneck. I don't have a gooseneck, but there's some added stresses added when converting a fifth to a gooseneck. I didn't see anything in two of the pdf's about gooseneck hitches, but didn't read the third.

 

Something else to consider is the length of your 5th wheel. If you stay in RV parks while on the road, some of them have length limits which can reduce your options.

 

Also found that an advertised 35' spot is often a motorhome spot and my 34' trailer won't fit. Also, my 34' is longer than a lot of the campgrounds I wanted to go to. Inside the Redwoods national forest was one of them.

Hard to find a spot the few times I went out. A lot of them are reserved months in advance. I always do find a spot, I just need to call around a lot and stay up to 45 minutes away from where I want to go.

 

i got the lomgest trailer i could find with a washer dryer and two cilling fans. 38'
got a feeling i will be doing a lot of boondocking with it :rofl:
my combined legth is too long to drive down the canyon from here :doh:

i needed something we could live in while we built the new house... seemed like a good fit at the time but now... it may be a tad larger than i would like for a lot of stuff but...
great guest house :hehe: if i can get it in my upper driveway off our one lane roads :look: