Here's what m-w.com has for the definition of:
Main Entry: muscle car
Pronunciation: -"kär
Function: noun
Date: 1969
: any of a group of American-made 2-door sports coupes with powerful engines designed for high-performance driving
My 89 GT convertible coupe has a powerful engine, 2-doors and was designed for high-performance driving. I guess Corvettes and Vipers do fit the definition.
The important thing is that the official definition excludes imports.
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351W 89 Mustang GT Convertible