AFR full cnc heads with the hard anodiize option, a cam, pushrods, fuel pressure regulator mod, some gaskets and find a shop that knows how to tune. Should cost around $3,500-$4,000 a side. This should bump you 100hp easy. If you want to squeeze another 8-10hp out of it add the harding marine crossover with the idler pulley. As mentioned earlier, 500EFI's like Whipples / boost, but plan on major drive upgrades as well.

The nice thing about the carb 500's is that you can always thow in a stroker crank without major changes to the top end, whereas the stock EFI manifolds don't seem to like more cubes.