It works with mods.
Trim down rear section where it goes under the back seat.
The bottom seat belt holes are slightly higher/set back a tad. Oblong lower seat belt hole where it bolts to the rocker panel.
94-98 Mustang front buckles were in the floor like a fox, but 99-04 Mustang front buckles are bolted to the seat, so if you get 99-04 stuff you have to poke two holes for the bolts under the console.
The sides where the sill paltes screw down are a tad too long as well. Just take some strong/sharp scissors and cut them out.
I have used 94-04 style carpets in two foxes now. Once all your trim is in it will look great.