OHC engines were designed before OHV. When Caddy launched its first OHV engine back in the early 50's, it was held as one of the most powerful and smoothest running V8's. Soon after Chevy launched the first Small Block, and the rest is history.
What made OHC engines "advanced" was the technology added to it such as Variable Valve timing where the timing of the cam or cams could be advanced as the engine climbed in rpm. GM and now the Viper have a variable valve technology on thier OHV engines.
OHC engines are also physicly larger then your average OHV engine. Displacement is just bore x stroke. The Mustang 4.6 OHC engine looks like it could hold a SBF inside it.
OHC and OHV engines have advantages and faults. OHC tends to have less noise/vibration/harshness due to less valvetrain parts. With that, its RPM band tends to be larger then an OHV. OHC also has great advantages with smaller engines, such as 4 and 6cyl. If you look at the new MB 6.2 V8, its a OHC engine that makes 505hp.
GM wont replace the Gev IV with an OHC engine any time soon, they have kept the pushrod, but remember they also have an OHC engine in the Northstar and upcoming Ultra V8 engines that will find itself in new Caddys in the next year or so.
As for a Twin-Cam OHV engine, GM already produced a concept of the engine called the XV8. Displaces 4.3 liters and was shown in the 2001 Opel Signum.
http://www.acarplace.com/brands/gm/xv8-engine.html