You cannot put a '68 or later Torqueflite behind a '67 or earlier slant-6 engine, because the '68-up torque converter nose and crankshaft flange counterbore are larger. You can use a later engine in front of an earlier transmission by just installing a readily-available spacer ring between small early torque converter nose and large late crank flange counterbore. But if you want to use a later trans behind an earlier engine, you have to make mods. You get a custom torque converter made with a small nose but the '68-up splines (different, so you can't just use the '67-back torque converter in the '68-up trans). Or you machine the crank counterbore larger.
Beyond that, there are hookup issues. The Torqueflites through '65 use a twin-cable shift system; one cable for the R-N-D-2-1 and the other for the Park lock. '66 and up transmissions use a rod/lever shift system. You cannot operate a rod-shift transmission with a cable shifter (buttons or '65 column and floor shifts), though there's a button-box rework service available that allows this operation. Also, '66-up transmissions have a splined output shaft to accept a slipyoke for use with cross-and-roller type front universal joint, while '65-down transmissions have a flanged output shaft for a bolt-up ball-and-trunnion front universal joint. So using a later trans in an earlier car would also entail a different driveshaft. Also speedometer cable would need to be the '66-'67 item.
