I let my fastest car loose on the salt flats after taking all the cooling off and altering the gearbox a bit (sacrificing low speed acceleration for high speed acceleration)
The end result is a 402mph flying mile, or 647kmh. It was still accelerating quite well even in the flying mile, with the speedo indicating ~410mph at the finish line
*edit*
with enough cooling to give me a positive MTBF I get 9.81s, which is 367mph and the limiter for this car with the chosen engine. I could probably improve that a bit by reducing the stroke and increasing the rev limiter though
*2nd edit*
I've just done a bit of googling, and the world record for the classes we are interested in* is 439.562 over the flying mile, so to beat that you would need a flying mile of 8.18s
*Category A;
Group 1 (piston engine with turbo);
Class 10 (5000-8000cc) and class 11(8000cc+)