I've had great experiences with Tremec's TKO600 transmissions.
I had one in my '69 Camaro, and it was exceptional. I bought the car with the transmission, so I'm not sure if the builder encountered any issues with it early on. However, while I owned the car the transmission was fantastic. I owned a 2006 Corvette convertible at the time, and I won't say the Tremec shifted as smoothly as the transmission in a brand new sports car, but it certainly was much nicer than the old 4-speed transmissions.
I also had a TKO600 in my current car -- a 71 cuda. The Tremec was in the car when I bought it. I had plans for re-restoring the car -- kind of personalizing it to my tastes, but I drove the car for a couple months before I started the process. It was certainly a relief having the 5-speed at that time, because this car had 4:56 gears. You can imagine how tight the car would wind. I'd have to drive the car around my neighborhood in 4th gear. Well, I was able to get the car on the highway at 60mph in 5th gear at comfortable RPMs (I don't know the exact RPMs since my tach wasn't working at the time, but the engine wasn't wrapping up or working hard at 60mph). I thought that was pretty impressive.
I've installed a new TKO600 into the cuda this time around too. This time, though, I've gone with 3:54s in a new Dana 60. I'm looking forward to cruising in it this Spring (maybe even a little this fall). I'm sold on the Tremec 5-speeds.
