I have Kenny Brown Super- subs that are zinc plated for improved corrosion resistance. Which ever way you go get or add seat braces as it helps spread the load and strengthens a weak spot in the Mustang floor.
After I did mine I found this site on the web. Just something to consider if you are or know a welder. If I had it to redo I would probably go for full length (firewall to rear frame) instead of the more common 48" ones as the install is the nearly the same and you get even more rigidity.
do-it-yourself subs (then click on "articles")