Look for a '78 that has clip retained halfshafts and u-joints, get the bowtie, mount,strap & strap hanger, the drive shaft and the differential. Then look for a 2nd gen turbo ZX, all you need is the diff (they are all LSD). Install the '78 cover to the LSD and swap the pinion flange for the driveshaft with the '78s. Install all the stuff you took off the 78 into your 240. You may want to check the breakover on the LSD, it's probably shot.