Big block or small block

Stroker small block. For the cost of all the big block parts you'll need, you can build a pretty mean stroker. 500hp isn't that hard to achieve with a 408 or similar.

As for cast vs forged, what's the use of the car? 500 hp is about where I would draw the line on a cast crank, and if its going to be a frequent racer then better to go with forged. But if its a street car that only sees the track infrequently and 500 hp or less, there's nothing wrong with a quality cast crank. Just my thoughts though, I'm not an expert.