Here are the components you need to change to upgrade a cable-operated transmission to accept a '67-and-up torque converter. You're looking at the input shaft/rear clutch piston retainer assembly and the reaction shaft support. These goodies are hidden behind the front pump cover in an assembled transmission, except for the two splined extensions, which are visible after you pull the torque converter. Beware, these parts come in two varieties: narrow bushing, used from 1967 to 1970, and wide bushing, used from 1971 through 1978. You cannot mix parts between the narrow and wide bushing versions. The component set must be from the same-vintage donor transmission. Your best bet is to scavenge them from an unmolested donor or source them from a competent transmission rebuild shop that's aware of such intricacies. Expect to pay around $100 for clean used parts.