You can still drive it if you have to. You just need to lay the carpet back down and bolt the seat back in.
I just did it to my '92 convertable. Let me know if you need any tips.
You can see mine here:
http://home.adelphia.net/~daver1/mustang.html
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'92 GT conv.
TFS heads, GT40, Powerdyne 9#, AOD, 3.73...