Oil and Transmission Cooler

You didn't specify if you are an automatic or 5 speed, but 50resto has a nice tranny cooler...

http://secure.50resto.com/product1.cfm?SID=1&Product_ID=2662&Category_ID=800

Oil cooler...
http://www.brothersperformance.com/shop/mustang/fmm6642s101.html

Throw some of this in while you're at it...
http://secure.50resto.com/product1.cfm?SID=1&Product_ID=2466&Category_ID=586

Then get a 180 degree thermostat too from any old parts store.

But why stop there? Get one of these:
7-294BC-FXX 1994-1998 3.8 Mustang Aluminum Radiator-5 Speed $399.99
7-294BC-FAX 1994-1998 3.8 Mustang Aluminum Radiator-Automatic $429.99

From...
http://www.need-4-speed.com/griffin.htm
or...
http://www.fluidyne.com/pl_hpdomestics.html

Don't forget to upgrade the fan as well...
No URL, can't find one that will fit - you look for one!

You can also upgrade the radiator hoses as well.
Again, you can search for something.

Also, read this article...
http://www.mustang50magazine.com/techarticles/33702/