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The pre-87 700R4s had 27 splines on the input shaft. They were less reliable than the newer ones. The newer ones had a 30 spline shaft and were of a much better design. Both could be built up to stand extra power. I would go with the new design as there seems to be much more support for it than the older design.
The 1988 is a much better unit to start with, and has many updates that are not in the 1984 unit. In mid 1984 they went from 27 to 30 spline. Go from there.
