Top Speed stock 5.7?

In short it will go as fast as you are and the car are willing to go.
there ya go- I expect with my mods, my Stealth would bury the 290 kph speedo. And no, there's no chance in hell I would try it.

As far as top end of an a-body with a 5.7, can't really answer that until gear ratio is added- there's a big difference between 2.xx gearing and 4.56:D