For many years, Leyland Motors (in the UK) made under-floor engined transit and long-distance single-decker coaches. Most had a 350 cu.in., 6-cylinder diesel engine and 4-speed stick-shift. The engine was laid on its side about midway between the front an rear axles. I think they made some of the long-distance chassis with their humongous 680 cu. in. engine to get a high-speed cruise on the freeways.
I can remember being passed on the freeway, when riding my Vespa, by 45' coaches doing 85 mph or more. I got a real lesson on wake turbulence!