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The boxes use different lines but the mounting is the same, i believe the input shaft splines are different though (different diameter and spline count).
GM did change the spline count and diameter i just today i looked up a rebuild kit for someone and it covers 80-87 and that's because of the input spline difference.
People swap sector shafts all the time to do crossover steering. It's an easy swap and straight forward, the hardest part is adjusting the sector shaft afterwards. Too tight and the steering won't go beyond center and too loose and your steering wheel will have too much play before the wheels start to turn.