honestly 500hp is not too far off with the 268 and a set of inertia big valve heads. We are talking flywheel numbers on an engine dyno minus the alt ac and and the other power robbing goodies. You won't find an aftermarket casting for the hemi head. And port design is critical you can easily kill the velocity if your not careful. (i sugest buying heads from someone)
now on the cam choice there are some fast (for a truck) trucks out there running the 268 but I personnly think there are better choices. I would call the vendors and discuss your plans and get a custom cam for your application. Then it can be matched to converter choice and will net you the most power for the dollar instead of a cam designed for a 5000 pound truck or a 4200 pound car.