i was told by several that the 63/64 fsj's had a hydraulic clutch
but that aside good luck finding one. I opted to use the 1980-86 jeep cj 151 iron duke and 6cyl setup. the iron duke setup uses an external slave cylinder that bolts up to many of the jeep bellhousings
specifically when i took it off of a 1981 cj7 with iron duke and 4 spd. when mounted it sticks roughtly 4" off of the firewall. Parts mike sells em for pretty reasonable.