US manufacturers designd and built cars for the US market, exactly like all the other worldwide OEMs. Back in 1970 I would argue that the global automotive market we have now was just barely getting off the ground. Manufacturers adapted (instead of designing) cars for foriegn markets.
In 1970 the US Interstate system was nearing completion and we had tens of thousands of miles of wide, fairly straight, and free roads across the country. Large, V8 powered cars are ideal for that environment and can cruise comfortably at 80 to 100 mph all day long. As a kid we made many multi-day driving vacations in my Dad's 1968 T-Bird and 1970 LTD.
No car is "better" then another car IMHO. Cars are all different, meet various needs, stir different emotions, and work as well as can be expected given their complexity and cost limitations.
BTW... Here's a picture of a Dodge Charger competing in an edurance race at Monza this year:
And here's what is sounds like when the Hemi is fired up:
http://www.olympia-charger.com/2007/09/3-2-1-ignition-.html