I've done both the Heidt's kit and adapted a 2003 Cobra IRS. The Heidt's kit requires lots of fabrication. It's a universal kit not designed specifically for the Mustang. It is available in a variety of widths so it can fit all cars.
The Cobra IRS is too wide for 65/66 cars without major flares. With the DVS kit it's basically a bolt in. Cobra IRS assemblies show up on Ebay all the time. I modified the cradle myself to allow me to lower the car further. Since I didn't buy the DVS kit it required lots of fabrication also.
Daze did a nice job with the Jag IRS but one again, much fabrication.
There are some complete bolt in kits available but they are around $9k
They're probably all more work and money than they are worth. Yes, they ride better and potentially can handle better but I doubt I'll do another one.