LA valve covers on Magnum heads?
Just have Comp grind you a custom camshaft. You can get whatever your want, and it isn't much more expensive than just buying a shelf hydraulic roller.
If you want to look stock, its worth it to get the heads drilled for the stock LA bolt pattern. You get a better choice of intakes that way.
I'm running some cast LA covers on Magnum heads with no leaks. I'm using the Moroso gaskets.
Steve