The weight difference would be maybe a couple of pounds, completely negligible. You add 3 cams but you lose all the pushrods and rocker arms. The CG would likewise not be shifted more than a few ten thousandths of an inch, which is again negligible and could be accounted for by changing the oil tank design or something equally as trivial. The engine wouldn't need to be physically larger at all. If the cams called for more valve cover clearance you could easily shorten other components to account for that, if you shortened the stroke you could rev even higher. Audi's V8 engines fit between the radiator and front axle, just as my V6 sits entirely in front of the front axle. They sit the 4.2 in the R8, and even get the 5.2 V10 from the gallardo and the V12 TDI engines to fit without any issue. I don't know why you think DOHC engines are all huge. It would make more power because it could flow more air and that flow is more efficient. Bam.