Had a shop that carried a bed I liked, install the flat bed for me.

Their install price was atrocious, but I was willing to pay for it because I had no way to do the install myself. They charged 3x what it cost to get my last flatbed installed. They had the truck for two days longer than quoted, did a poor job on several parts of the install, and didn't do a lot of what I asked for.
On the other hand, they stayed until 9PM on Friday night because I had my wife drop me off and told them I wasn't leaving until my truck was done. They may have taken more time and done the rest if I weren't rushing them, but they already were way over the time window they quoted.
Notice in the above photo how the toolboxes were not installed flat. I had to build spacers and correct that yeasterday.
They also installed the bumper...

... but didn't trim the fender liner to match and didn't install the winch like they were supposed to. Other than that, the install looks pretty good... especially since both Ranch Hand and the shop said the 3500 bumper wouldn't fit the 5500. The only thing that didn't fit were the tow hooks, so I ordered a set for the 3500s which will clear the bumper and fit. Those should be here in a few days.

Like for my old truck, I ran angle down the sides of the bed with 51" between them. I decided to change it up and use an extra mat to have a full 51" of coverage because my camper sometimes was sitting off the side of the mat on the old truck. I shim the camper base (49" wide) in with 1" plastic blocks I cut from Camco bricks. Also mounted the spare on the headache rack and put some dock bumpers in the front.

Got the camper on the truck, upper toolboxes reinstalled, then started working on the tie downs. Ended up just drilling hles in the rub rail to put them where I wanted. My tie downs are homebrew fastguns, that use threaded rod which is great because I can cut them to the appropriate sizes.
The top toolboxes are 48+18" on each side, bottom ones are 48+24" on each side. In addition, so much deck space...

My fix for the camper (see earlier post) took out all but about 1/8" of the sag, and I was able to use the same location to lock in the 2kW generator and gas can I carry (in addition to the factory onboard) with the tie down as a theft deterrent.

The 18" extension I bought was perfect for towing the Jeep with, and I sat in the Home Depot parking lot for an hour making lots of noise. If you look back up at the photo of the generator storage, notice that I cut arches above the tires that will give me better clearance when the 37s go on. I may need to weld in a radiused sheet to soften the edge if the tire hits, but it's better to have the clearance and if needed, I'll deal with that once I have the tires fitted today.
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The shop also failed to install the 7-pin connector on the headache rack, backup camera, and parking sensors that they agreed to install, so I installed the sensors and fabbed up a quick bracket for the camera and got that on yesterday. I'm not sure if I'm just going to use a splitter or bother adding another 7-pin myself yet.
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We moved about an hour up the road yesterday, and the new truck feels way more stable than the 3500. We're happy we made the switch.