I added a 5-speed to my '79 rabbit and the only bit I recall not working was the selector lever which is longer on the 5 speed in order to fit the extra row side-to-side in the shift layout. I cut it and welded in an additional section to make it a little longer than the stock 5-speed selector lever. It's worked great for a couple years without issue. Roger Brown sells a bolt on version if you don't have a welder. I remember I replaced the bushings and linkage rods at the same time a couple years ago and so there might be more to it, but that was the only 'special' item I dealt with.