Personally I like the 518 option better than the gear vendors, because for one the total cost of a 518 conversion would be less than the price of buying a gear vendor unit and still having a non rebuilt transmission (unless you have your existing transmission rebuilt at the same time, which adds more cost), 2nd if something goes wrong with the 518 compared to the gear vendor parts are a little easier to get since the 518 is a factory transmission compared to an aftermarket addition. You can get a 518 with a lockup converter which acts like another half gear to aid in fuel economy. Granted you do have to make a custom crossmember and slightly modify the transmission case's extra webbing to make it fit but that's it. The install is pretty easy and straight forward.
Check out this link they sell rebuilt 518's and they sell the parts to install one in your car. Do not listen to people that say you cannot use the stock slap stick linkage, because a member on here did it and I think the pictures and details are listed in the thread linked above.
http://www.redrider.us/518_transmission.htm
