I don't have any idea what the motor made before, but something around 500hp certainly seems about right for that combo.
Sidewinders don't flow 290cfm ootb, but can pretty easily without too much work.
Although, even with them flowing 290, that combo on our dyno would be very hard pressed to make 590hp.
I've built and dynoed several stock stroke 440's over the years that approached(or exceeded) the 600hp mark, and they all had quite a bit more duration than the OP's cam.
Depending on just how good the heads end up being, and what the final CR is, I'd say more like the 560hp range is more realistic.
Which....... If the car was set up properly, would get it into the 10's.