Are you talking about the early-mid 70's capris? In '79 the Capri was changed to be a mirror of the mustang but before that they were V6 powered, light economyish cars. I don't think i've ever seen one in person, but then again i've never really looked. In pictures they look pretty damn light, so i think you're right about them being very light weight. According to alldata those capris were made from 1970 to 1975 and the capri II was made from '76 to '78. I don't know the differance so i can't be of much help. Basically anything that lightweight with a solid V8 can be made to compete with Camaros and Corvettes, but it all depends on how it's set up and how you're racing (drag, autocross, road racing, etc.). And yes according to Alldata, they came with the 2.3L 4 and the 2.8L V6.
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'84 Mustang 5.0 LX
My car