The 231 has many versions, there is no one-size-fits-all 231. Some mixing and matching of internal parts is possible. You need to be sure of the input spline count and of the internal gear cut if you plan on any internal part swapping.
Because of differences in the cut of the gears, as a general rule, one should not interchange 1987-1993 NP/NV231 gears with 1994-present gears. These years are not always hard and fast delineations. If one is not absolutely certain, disassembly and inspection is required.
Novak Adapters has some excellent Tech info -> http://www.novak-adapt.com/knowledge/np231.htm
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Originally Posted by Ronald W. Reagan: Government is not the solution to our problem; Government is the problem.