High performance (power) don't know.
Have had personal experience with 3 sixes; one Chevy, one Ford, one Jaguar. Chevy 235 was a lemon, and probably not representative of the genre so will not discuss it. Jaguar 3.4 was powerful, fairly fuel efficient, and an absolute bear to work on. Didn't keep it long enough to really comment other than the above.
Ford 300 averaged about 22 MPG at highway speeds in an extended wheelbase Ford Econoline with 4-barrel carb and 4-speed transmission. Lasted more than 500,000 miles until the body rusted out the 3rd time, and I gave up. Should have rebodied the drivetrain rather than scrapping, as I did.
Jon.
