Do yourself a huge favor and call either Mustangs Plus or Eaton Detroit Spring and have them match a set of springs to your car and how you plan to use it.  There were probably 20-30 different springs each year for the Mustang based on engine, body style, rear end, special packages, etc.  Sure, a universal spring will fit, but how will it RIDE?

Most guys on here are going to steer you towards a 4.5 or 5 leaf spring.  Most feel the standard 4 leaf springs are not adequate for having the car full of people or proper handling. 

Shopping springs based only on cost is a sure fire way to be disappointed.  The suppliers I noted above are ones with whom I have personal experience, and I know offer quality products.  Most of the cheapie springs are made off shore with questionable metallurgy and heat treating. 

The cougar springs are 2” longer.
Checknout the RIDE HEIGHT PAGE link atop the forum. The info is pretty old, but you can get an idea of how the car will sit.
I prefer 4.5 leaf reverse eye from Mustangs Plus if you have alot of hp. The four leaf reverse eye if 300 hp or less.