A few things -
1. choice of mufflers is limits due to where they need to be located
2. Pain - not alot of places for mounting points. You can attach to the lower edge of the pinch weld but you'll have to support the other end of that pipe in at the driveline tunnel to keep them from turning (a problem early Shelbys had )
3. There must be kits (?) but we hand built the system I did from factory details (65 Shelby)
Would guess guys like Jim Cowles either have or will make up a system
Good luck with the project
Jeff Speegle
MCA Shelby Gold Card
http://www.geocities.com/MotorCity/6473
Mustang & Shelby research, documentation
http://albums.photopoint.com/j/View?u=1609818&a=12284202&p=44925357&Sequence=7&res=high.jpg