The smaller FE motors are fucking tough as nails. It will take a 250 shot, and laugh at you. The biggest thing is to get air to it. Get a vacuum gauge, and make sure you aren't pulling vacuum at WOT, if so, find a bigger carb. You shouldn't need more than 600cfm really. Just bolting a carb from a 460 on will probably hurt you more than help, the jetting would prolly be too much. Find a similar size engine in a junkyard and rebuild the carb. You have access to a wideband? If so, you could just re jet the 460 carb accordingly. Then address somehow getting those shitty manifolds off. Any way possible.
On a sidenote, I want to see a schoolbus smoking one pair of rear wheels when you hit the spray.