Doug captured most of it for the gasket and seal kit. If you buy the master kit with frictions and steel separator plates there are thickness differences from the early to later MY units - I forgot where the break was. If I recall correctly the early were thin then we went to thick on both the steels and frictions. The later design had greater heat capacity so they were less likely to potato chip from overheating. There were some design changes along the way regarding seals - primarily for wear resistance and cold weather operation and to prevent heat aging - the better kits, maybe all at this point have those improvements.

If you are positive no one has taken the trans apart and updated it or made a lockup out of a non or vice versa - then order the kit for your MY application. The later style feeds the rear clutch though the rear seal on the input shaft and a seal on the rear clutch piston support where the early units fed the rear clutch through the two seals on the input shaft. Many guys forget that large seal on the rear clutch piston support - you get a no drive condition and sometimes a drive/creep in neutral condition. All I can say for guys doing one is go slow - take pictures, match seal in the kit against where it lives in the trans before

throwing out the old ones. Nothing more disturbing than getting the trans done and find a seal left over without explanation. The correct year service manual is usually helpfulll too.