Your horsepower sounds like a reasonable guess. I'd assume this is a flat tappet setup so any max lift the cam makes should be divided by 1.5 and multiplied by 1.6 if you use stock ratio Magnum rockers.
With stock Magnum heads there are a few clearance items of concern, what intake will you run?
Other folks on here have done it too though and it's fairly documented. I have all stock accessories, stock intake, and stock valve covers on mine.