Yes you will have to shorten your rear drive line. The good news is that the T18 (close ratio, 4.03 first gear) was offered behind the I6 in a 1976 CJ5 so it is an easy conversion. The 1979 CJ T18 is the wide ratio, 6.32 first gear and has reverse over and up as all of the others were over and down. There are a few things you need to do such as lower the skid plate and add some spacers between the transmission mount. Try this page: