I believe all Clutchflite input shafts are made in the pre-'78 2-ring configuration. In '78 they reconfigured the input shaft and it's relation to the rear clutch piston retainer and the reaction shaft support.

To make the Clutchflite work in a 518 you'll need the pre-'78 rear clutch piston retainer, the Clutchflite input shaft and a pre-'78 reaction shaft support (pre-'74 is most desirable). Then the only problem you'll encounter is the bushing in the rear of the input shaft, it's bore is larger than the pilot shaft on the transmission intermediate shaft so some machine work in the form of a reducer bushing will be in order.