Thanks for the reply. I have been talking with Greg from Electro-Tech, Inc. and he is puzzled also. Seems as if this model had its own unique speaker installation. It is a standard 4x10 speaker size common in 60s GM cars, but the installation is much different.
I remember taking the original speaker out 25+ years ago (which is long gone, of course). I remember it just clipping into that wire harness/cradle.
I'll keep my eyes open for a OEM speaker, or try one of Electro-Tech's 4x10s and get it to fit.
Steve