RE: switching from a 3 speed to a 4 speed?
As stated, you're ahead of the game by having a manual tranny car. The shift linkages will NOT work. As far as driveshaft length, that shouldn't be a problem, however the only hiccups you might encounter would be finding an early sixties 833 that has a ball-and-trunion yoke as opposed to the u-joints we are used to. Little things you don't think of will drive up the price, such as the reverse light indicator, the flywheel mount, and other small things. Good luck with it. Instead of forking out the big bucks for a Kiesler, I would go find a "boneyard" (if that quote could applly here) T56 six speed out of a Viper, Camaro or Vette.
Later
KFD