This is completely silly, but fun, so i'll answer.
You wont be fording however, the water displacement of a 'hull' that size will actually make it float so the snorkel need not be excessively tall. Actually the normal air cleaner will probably work just fine under the hood. You'd be building an amphibian instead of a submersible. To build a submarine version would require a ridiculous amount of weight to stay down equal to the displaced water, a cubic foot of water is 62lbs so a back of the napkin calculation of 40ft long by 8ft wide by 8 feet tall is already 2560 cubic feet or 158,720lbs of displaced water(!) Something of that weight wouldn't be road legal, not even the 4 axle Michigan quads (26 wheeler) for rock hauling weigh that much and wouldn't even have a strong enough frame for it let alone axles or suspension. Plus extra weight beyond displacement to actually have traction while under water. So you'll be building an amphibian.
Look into the Terra Wind amphibious motorhome. It's been done.
If you wanted to try and clone it, first step would be to take the entire body off and obviously waterproof it to a reasonable depth that would float the whole vehicle, maybe up to the bottom of the windows to be safe. Sealing everything with fiberglass, a replacement higher mounted or water proof door and making sure all penetrations of this hull are done like a boat would be designed for underwater use, mostly the steering wheel as the only essential thing that cant be put higher up.
Then your powertrain will have to be waterproofed. Many military trucks are made for fording so something like parts from an M35 would probably make a good replacement powertrain.
You'll also want some kind of a jet drive probably to actually get anywhere in the water.
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