I finished a TKO installation in my '70 Charger last month. I could have bought the TKX but the benefits it provided did not appeal to me. I got a great deal on my kit.
I went close ratio with the .64 Overdrive. THIS changed the feel of the car dramatically. Cruising at freeway speeds, the car is quieter, smoother, no vibrations....it is so much easier to drive fast. The 500 mile break in sucks though.
I had to cut the trans tunnel to make room for the top of the trans but SST provides a template and a patch panel. This was the only 5 speed swap that I have ever done and it went well. I wrote a multi page thread about the install at FBBO.