Here is my opinion. You are doing yourself a disservice HP-wise by sticking with manifolds. I understand the argument for manifolds on the street. They're cheap, reliable, and you already own them. That, and headers are known for being a PITA. Then there's the installation, the scraping, the leaking... blah

What is the argument FOR headers? properly sized, better HP and toque all across the range. In your scope, for something around 500HP, you're almost assuredly giving up 40-60hp (this is dyno proven) by not using a good header and appropriately sized exhaust.

So the question becomes: do you care? if not, stick with what you got, and you'll never have to worry about exhaust until rust eats a hole in it somewhere. If you want the reliability, don't hog out the outlets for reasons stated. You won't feel the gain, but the cast iron will feel the loss. By all means gasket match if you feel there is enough meat there, and take off the sharp edges. beyond that, I'd live with them as-is.

or find some of those nice max-wedge cast iron headers