Hp number idea on new car

Ok so I've kind of got this car going, seems to be a new issue every time i drive it though. Anyway, it's not really as fast as i had hoped......

What would be my next best bang for the buck? Trick flow 240 heads and a different cam? I'd like to be more like 600+ hp. Is that possible from the 440 or will i need a stroker kit?

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