I would never waste even $600 on new headers for a WBody.
SD's are just fine...and 50-100 bucks worth of modifications and they're just as good.
Get better clamps, RTV the slip joints, and replace the crappy flex/gooseneck downpipe. Done and done.
You're not going to make any more power with ZZP headers over SD's anyways...unless you're looking for dyno numbers and you need more flow...then try tracking down some old TOG X's.
IF you can find them.