They would, but of course it's going to cost you.....likely way more than you guys think.
And good thing nobody makes a fiberglass replacement panel, rockers are a key structural component in any vehicle, and replacing the high strength metal with fiberglass would be highly unsafe.
I've seen plenty of aftermarket rockers out there for various vehicles, stands to reason someone would offer one for the 355 platform. Still going to be expensive to install though.