FYI the 65 GT350 did not have insulation under the package tray . Owner added padding is a alternative. A tight spare tire will hold the tray firm but even with padding the tire itself will squeak somewhat flexing on the fiberglass .
Some people love the sense of being inside of a base drum with someone beating on it. It adds character.
The tire does squeak on the deck. It will wear a pattern into the gel coat.
Considering how much of a pain it is to get the spare tire out and considering that your life in a hard dead stop is dependent on how well that j-hook holds the tire in place, I personally would prefer a very light weight facsimile of one back there.
I remember Dick Smith saying that the race tech people were giving him a hard time with the batteries behind the seats in his S/C. I'm wondering how they would feel about the original 65 GT350 spare tire mounting? 30 years ago, "They" told me that they wanted it BOLTED to the floor with 5 big hairy 1/2" bolts.
I remember a couple of GT350 owners locating the battery under the deck but I'd prefer to keep my shotguns under there.