Not sure,but I cant see why you couldnt.You can try Paddock.com and look under mustang,they sell a ton of restoration parts 64-04 I beleive.I looked in my Texas Mustang Parts Cat and they dont list anything seat wise above 98.
Maybe instead of replacing the foam hit up ebay or craigslist for the whole seats,The GT's leather set should be $350-400+,worth looking into.Last month I found the entire charcol leather set for my 01 gt for 425 on ebay(I accidently ripped mine...DOH).