Not allot of pictures for non-Mach I examples (since these include Shelbys and Boss's built at Dearborn also) but found enough to provide a picture of what was done in this matter
Basically it appears that the sound deadener covers just the gas tank as expected then forward up and onto the rearend hump of the rear floor
Just a little of the sound deadener peeking out in this one but the picture shows its there
And don't forget the spare tire board
Hope these answer your questions