Well if its going to be 16' long max then 2x8 windows will not work you'd need to go about a foot longer for space to adhere the viewing panel. You will need roughly 4"/panel. I guess you wouldn't need a foot longer but you get my point, maybe just 6". So if 16' I'd say 2x6. One of the only determining factors in glass thickness is height of the aquarium or more so the depth of the water. My 180 is half inch and its 24" deep if you are having a true 36" depth id guess 3/4" but like I said 3/4 its a guess. Either way you go make sure you put bracing from front to back at least every 2-3 feet. There are glass thickness calculators out there don't worry about the fact it about all glass tanks same rules apply.
Make sure you make a build thread I'd love to see this monster come to life!