I hate to be the bearer of bad news, but with 1986 heads, you're never going to see 300 horsepower. Those are THE single worst year for performance of the 5.0L, bar none. You may also want to find an '87-'93 block for a little more strength, and a rotating assembly from '87-91.
Anyhoo, a '66 with the kind of setup you're running....um, mebbe mid- to low-14's. We don't know what gears, intake, cam, shift kit, etc. you're running, but I can absolutely say that those heads are going to be the limiting factor.
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Capri306, Moderator, The Mustang Works Online
"PROUD TO BE AN AMERICAN!!!"