I thought you could put a cam snout extension and an LA timing cover on a magnum? That would allow you to have a mechanical fuel pump while keeping the stock cam. Although for performance reasons, regrinding the cam would be a good idea which would still be cheaper than buying a new cam. Installing a mechanical pump seems like less of a hassle than an electric pump with a return and regulator.
I've never messed with a 5.9, so I can't speak from experience, but I'm pretty sure you can make a mech pump work with the stock cam.
Link to cam snout extension:
Hughes Engines