Personally, I never heard of any lightweight F-body options. I'm pretty sure that a friend who races an Aspen in Super Stock would have mentioned it with all the discussions we've had about Aspens/Volares since I bought an Aspen wagon from him for NHRA/IHRA Stock. He worked at a dealership in the '70's & '80's. But I'll ask, just in case.
Did you find any "Feather" parts to install on your '75?