Good news is that while there is a front to back brace under the sheet metal, it's 2 1/2" wide, and I had no issue mounting an NMO mount in the center of the truck roof.
First step, remove the rear dome light:
Use a small screwdriver in the small slot on the back of the lens to release it.
Next, use the small screw driver inside the two slots near the back of the dome light assembly. You need to gently release the two tabs to let the assembly be removed.
Second, remove the assembly the rear dome light attaches to. There are two tabs near the front that need to be depressed to allow the assembly to be moved. You've got to work it around above the head liner, but once you get it out, you can easily see the 2 1/2" wide rib with a nice hole to route the cable out of.
Once you have the dome light out of the way, you can drill the 3/4" hole for the NMO mount. There are indented ribs on the sheet metal roof skin, but there are two places where they flatten out and are flush with the rest of the roof. I put my mount on the rearward one of these. It puts the antenna just behind the "B" pillar. A layer of 2" wide masking tape to protect the surrounding paint was put down. I measured a few times to be sure I had the exact center. I did the deed with the appropriate 3/4" hole saw.
After drilling the hole, I fed the coaxial cable rearward down the channel towards the rear most dome light. The hole in the channel above the headliner is big enough that you can get a finger in there and catch the cable. It's pretty tight, and I ended up using a hook to catch it and pull it down. I pulled the rest of the slack through and mounted the NMO mount.
I used a 7/8" grommet in the hole above the head liner to protect the coaxial cable.
At this point, you need to loosen up the head liner in the back of the truck. There are two buttons that need to be pulled near the rear window. They just pull straight down and out. I removed the screw cover over the attachment screw on the passenger side "C" pillar trim. You'll then need a 7mm socket to take that screw out so you can move the trim out and see what's behind there. You can also get a good look at whats in the way along the right most edge of the head liner. There are curtain type airbags on my truck, so I had to be very careful how I routed the coax.
I then routed the coaxial cable towards the rear of the headliner and over to the passenger side "C" pillar. I used some gaffers tape along the way to hold the cable and keep it from sliding it around. If you don't know what gaffers tape is, it's like duct tape on steroids without the shiny coating. I have a big roll of black stuff that I use all the time. I used it all along the coaxial route to keep it out of the way of anything sharp and limit any rattles, etc. The one place you really need to be careful is at the rear most corner of the cab. The gas generator for the air bag is down low in the pillar, and there is a metal pipe that comes up to the air bag. If you are careful you can route the coax between the back corner of the cab and that pipe. I used loom at this point since there were some sharp edges.
Once around that, I brought the coax down the "C" pillar. Beware of the seat belt tensioner. I'm mounting my CDM1550 behind the rear seat on the passenger side, so I ran the cable under the rear trim and make sure it was in split loom.
I ran out of time, and will do the power, control cable and mount the radio and remote head next week.
Here it is with the VHF antenna installed:
Hopefully this will save someone else some time. Looks like you could mount a few antennas along the center line if you needed. Offsetting them would be easy too, but I only needed one and didn't want to pull the whole headliner down.
