Welcome to the Corvette Action Center Community. I'm sure someone here is knowledgeable about Hooker Headers. They'll respond to your questions. Why don't you try HookerHeaders online, do a google search, might answer your questions.
I've had mine apart several times and switched from the reverse-flow to glasspack. I am surprised you need to replace them.
Be sure to reverse the glasspacks so the scalloped perforations are opened towards the rear of the car. Slightly louder but much better flow.
As far as removal is concerned....
Spray wd-40 or similar product in excess around the seam of the header and sidepipe. Let it sit overnight.
Undo the two frame mount bolts and pinch bolt . Use a screwdriver to "unpinch" the small split by the pinch bolt.
Put a large screwdriver through one of the frame mount bracket holes so that the shank is resting on the pipe and push down. This should "unfreeze" the pipe from the collector. Just keep this procedure up with a little back and forth wiggling and they should seperate easily.
The inserts just drop right out.
I did this with HD baffles in straight pipes ... not perfect but it might help in the future. To help PREVENT this ... try some ANTISEIZE PAST on the baffles' edges before you re-install baffles. The good stuff is rated at very high temp. Essential for many other uses, a one pound can'll last most hobbyists a lifetime. My 3-pounder is at least 25 years ... can't read what's left of the label.