Let me start by saying I HAVE NO IDEA IF THIS WILL WORK. I found this bracket, and it looks like the ones 2009_FLTRSEI3 was offering, but chrome plated instead of stainless steel. I would gladly have bought one of his brackets, but it seems they were a "limited production" item. I just ordered this bracket from J&P Cycles for a grand total of $19.73 ($9.99 for the bracket, $1.75 handling and $7.99 shipping) Although I think the shipping and handling fee is excessive, it will be worth it IF this fixes my gap issue.
http://www.jpcycles.com/product/820-195From Mustang Website: (This makes me think the holes for the bracket will line up properly)
Most Mustang seats are designed to install just like your stock seat--
using the same mounting points and hardware as your original seat. It usually takes about fifteen minutes to remove the stock seat and install your new Mustang seat.