Well.....the reason a 1980's Volvo 240 would spoil the mood is that it's not representative of the time and place. A Mustang SVO or Shelby GLH would probably be irritating too, even if they would be nice for another game. Redline was one such game that I wanted to build these for and never got to it.
However, the reality is that VW Beetles and Transporters were very common in 1950's-1970's America, especially in California. A 1974 Porsche 930 Turbo would be going too far, but a 356 (or very early 911) would be representative, and would give the opportunity to mod the Beetle or Transporter, provided both were counted as being from the same manufacturer.
Also, nobody has said whether an A/C Cobra would be appropriate. A 427SC originally cost $7000, making it expensive, but comparable to a Corvette L88. Besides, it was also the most famous work of Carroll Shelby. The Mustang Cobra was named for it, after all, not the other way around.