Hey, I would like to put a roof rack with Hella lights on top of my Blazer. I like the baja style that covers whole roof. My concern is that the roof rack + cargo on a highway might put a lot of stress on the shell. How would you approach that? Below are examples of what I'm trying to copy.

 

My full-vert '74 has a home made roof rack on it. I carried around a 33x12.5R15 tire on it for a bit without issue, but I didn't put a lot of miles on it with the tire up top. What I did drive it was back roads & offroad as I use that truck for hunting purposes so it was kind of extreme use. I never put more than ~150lbs of stuff up there though. Any more than that and I would worry about the structure of my top as it isn't all that great to begin with.

If your top is in good condition, it likely can handle 150lbs easily. You'll likely want to run at least one set of the mounts into the steel roof. If you're making it yourself, you could run two sets of mounts into the steel roof and then a third set of mounts at the back into the fiberglass top.

 

I thought for some time, that these roof racks are supported by roll cage or some sort of metal/fixed structure that sits right under the fiberglass. Yes, I can definitely use steel roof for 1 or 2 sets of mounts, that's a good idea.

 

I have a 4 post 'family' cage in my '74, but the rack just bolts to the fiberglass top. All of that was done long before I bought the truck.

The only problem with mounting to a cage is how everything moves. In my full-vert, the top moves around a lot more than cage in the back does. A full cage with mounts at the 'A' pillar would limit this and your truck having the steel roof over the front row will have less body flex than my truck anyway, but worth noting.