I understand I'm late to the thread but here's what the original superbird scoop hardware looks like. I would say theres nothing really there keeping it in place other than stripped out plastic.
Agreed, just years of rust and embedded old Cosmoplast/FRP (fiberglass re-enforced plastic) stuck in the rusted stud threads, if the OP said he could get a set of vise grips on the stud, then there might be enough stud exposed to use a drill, tighten the chuck on the stud, go in reverse and spin/pull at the same time gently/slowly as to not smoke the FRP/plastic, you'll blow the hole out getting the stud out, clean up the hole with a small dremel using a burr to get to some rustfree/clean FRP, then use a bolt 1/4-20 thread, wrap the threads lightly with one wrap of thin white teflon tape, you could use an epoxy/JB Weld/etc, I like to use polyester resin, fill the hole, push in the teflon wrapped bolt, wipe away the excess resin/epoxy/etc, let dry fully, unscrew the bolt, you'll have fresh threads, I always replace/use Stainless 1/4-20 set screws studs and stainless K lock nuts....
