If you simply must have the first version of the Mustang in a convertible and have $25,000 or less to spend on a nice one, well then you’re likely to have to settle for the six-cylinder model instead of the 260 or 289 V-8. And the six of the ’60s was nothing like today’s 300-plus hp base V-6. Nope, the 200-cubic-inch straight-six was good for just 120 hp, so performance was leisurely at best. No