now about that roof deck
In my mind, the best place for mounts are inline with the rivets. Those "beams" or roof "hoops" if you will are the strong points.
here is how I would do it.
learn how to use "riv nuts" watch videos, then buy some and practice mount some... use 3/8" , now when you get to using them, drill with a square faced drill -- black and decker "bullet point" look it up... anyways drill that first part through the sheet metal. then drill with the correct bit the size hole needed for the rivnut.
here is why to do that way....
as the bus travels, the sheets of steel riveted to the roof ribs, move in relation to the ribs. With enough time the rivnuts will loosen because of this. so I believe you will have a longer lasting set up if the riv nuts are only in the ribs.
put locktite 660 on the outer part of the rivnut before installing
use anti seize compound on bolts before installing bolts.
so what you have now is this threaded thing that is riveted to the ribs on the roof of your bus.
I am going to use one row down the middle, one row off the right, one row off to the left.
Now, go look at internet information on something called "unistrut"
three rails of unistrut will use the threaded insert/bolt system to attach to the roof of the bus. The unistrut runs front to back.
I would mark six points on the roof. use grease pencil. three points in the front part of the deck, three points in the rear of the deck. Snap a string line. That will give you a guide to keep all the bolt locations in line with each other in the front to rear direction.
As you assemble, the process will be some thing like this, center punch bolt location, drill hole in roof sheet metal, change to smaller bit, drill hole for insert,put locktite on outside of insert, install insert. do this for all the holes in one front to back row. Force, using your fingers, seam sealer into edges of the sheet metal. leave the seam sealer a little proud, that is, raised. You should now have a threaded hole surrounded by a volcano of seam sealer.
when one row is done, bolt one of the unistrut rails to the roof. The bolts will have anti seize on the threads. Important when the time comes to ever undo this with out spinning a rivnut in a hoop. It really sucks to have one of those inserts spin round when you try to unscrew a bolt.
once you have three unistrut rails on then the next step is to attach decking to the unistrut rails
Go learn about building with unistrut and learn about putting nut inserts ...
I hope this gives you a better idea of what you are looking at after you do some skool work.
william
