I should have said that most American sports cars at that time were better than the rest of the world's, not specifically the 390.
For instance, the Corvette had 360hp until 1963, then it had 365hp til 1965 when it got 375hp. The Stingray could be bought with an advertised 430hp but was alleged to put down more than 550hp.
All these motors were fuel injected, light years ahead of the Brits. And a curb weight of 2700lbs. 4-wheels disc brakes on the Vette standard. Air conditioning, AM-FM radio, telescopic steering wheel, fully independant suspension, etc.
Then look at the Mustang and Camaro which destroyed everything in Trans-Am racing.
Then the Daytona Coupe and Shelby Cobra that dominated Le Mans. The GT40's chassis was designed in England (it lifted at high speed and was scary through the corners as well), but the power plant was all American, and it still remains one of the great engines.
Then there's the Thunderbird, classic American sports car available from the beginning with a 5.1L V8 and a curb weight of 2525lbs.
Sure the Brits had no shortage of sports cars, but none of them could match a Corvette in terms of performance.