I've got the green 700R4 "update" manual which shows what changes were made to the later years, and for that matter what retrofits.

It shows where the turbine shaft on the input drum was changed to be thicker (30 spline) but ALSO shows that GM made a "retrofit" shaft that has the orignal 27-splines and fits the early 82-84 transmissions, but is beefed up where it needs to be. Apparently the weak part isn't where the splines are, but where the seal rings are.

The book says the part number is 8647963, but I can't seem to find it..