as the question states....is there any?
Of course there are.
Go outside, open your hood, and look down. :D
There are aftermarket underdrive pulleys for the SN95 Mustang with the 3.8. The belt routing is slightly different though. You could do some research to find out the size of the SN95 underdrive pulleys and see if they will work although with the different belt routing you would have to measure The route of your new pulleys (if they will work with your setup) with a string. I'm pretty sure the belt that would be recomended with installation on a mustang would not work with an older 3.8 serpentine.
Fox Cougar/T-bird belt routing:http://autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/14/cc/ef/0900823d8014ccef.jsp
SN95 Mustang Belt Routing:http://autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/1d/a4/de/0900823d801da4de.jsp