The build sheets I have acquired over the years have all indicated 5 wheels/tires, I have never seen nor heard of build sheet information indicating 4 wheels of one type and another wheel/tire for the spare in 1968. The MCA rules state that the spare tire can be either the styled steel wheel or standard steel wheel, which continues to fuel the debate and confusion.
I have documented numerous 1968 GT's with the incorrect (original) spare tire storage stickers. The MCA rules require the sticker match the wheel/tire combination or you may receive a deduction. As an owner you have to decide to leave the original sticker or replace it with the "correct" sticker.
To the best of my knowledge the felt mat started in 1969 for the Mustang models. I don't know of any supporting documentation to indicate it was used earlier than 1969 for Mustangs. The jack and lug wrench are stored under the spare tire for the styled steel wheels and the mounting plate, unique to the 68's with styled steel wheels, was used to secure the wheel/tire, jack and wrench.
Danny