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You might be able tuck the tank up under the expanded metal that you walk on when you connect a trailer to your tractor. I call it a catwalk. I am sure there is some official name for it.
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If you put it on the passenger side of the truck, you won't stumble over the bolt heads. Suggest using threaded rod. to affix the tank and maybe a metal plate so the bolt won't slip through...
You just want to be high and tight somewhere. If you spot a better place like behind the step - that could work too. Just don't be stingy with the zip ties to hold the nylon lines close - so that don't get caught in a branch on the ground or something goofy. Be sure to get you some good DOT Approved line to tie the tanks together. Bigger is better for volume.
I am not certain, but a lot of those multiple horn configurations use the one stream of air to power all of them. I have a 5 trumpet set that will rattle your brain - but the 36 inch single trumpet that I have seems louder. It is a lot deeper sound too. Almost like a fog horn. They are a lot of fun to have for sure!