Six conversion info:
http://www.pelicanparts.com/914/914-6_construction_kit/914-6_construction_kit.htm
Doing a decent-quality conversion is not cheap, and it isn't trivially easy. It's not (generally) a difficult nightmare, but it's not the "bolt it up and go" procedure that some outfits would have you believe.
On what driving an original six is like:
Really really nifty. It feels quite underdeveloped--you can tell that there is a whole lot more potential in the car that is not realized. The motor makes all kinds of really wonderful racket (six thirsty Weber throats and a big-ol' fan right behind your head!!) and it will indeed get out of its own way. Compared to a 914-4 with the optional sway bars, it really leans in the turns and doesn't hold on as well. Stopping is quite good, though some of the "classic 914 mush" is present. Shifting is usually between the tail-shifter and side-shifter in feel, except when the bushings are worn. In which case it's the familiar crapshoot--move the lever and pray... The interior is the early one, and the non-movable passenger seat makes it feel kinda cheap, as do the manually-adjusted seat belts. Not sure on the windows, as we never bothered rolling them up on the drive.
I've also ridden (student at an autoX) in an original Six that had a well set-up suspension and a slightly tweaked (external mods only) motor. That was much more impressive, it felt like a slightly-heavy 914 that would get up and
move when asked. Plus all that wonderful racket...
In general, an original Six is like an early Four, but with the motor, shift linkage, and brakes improved. Uhhh, 'cause that's almost exactly what it is.
--DD