Either the engines are at the end of very long intake ducting or the engines are exhausting through very long jet pipes but either way a lot of power would be lost, theres a reason why the first gen jets had podded engines with minimal ducting. Nowadays with 70 years of wind tunnel testing and computer aided design we still dont see many aeroplanes with long tortuous ducts or jet pipes.
Also I dont know if its the picture but those wings look horribly thick. 600mph with wings of a similar profile to a P38 nope nope nope.
Consider the F-100 and the F-8, both of these 1950s fighters had long intake ducting. In the case of the F-8 it was very long more than 40 feet. They weren't the only planes with a long throat.
It's hard to say about the blended airfoil. Wind tunnels tests would have been nice. I'm guessing it could have reached 600 MPH true airspeed at high altitude level flight. Maybe more. I really wonder how it would've behaved in the transonic regimen.