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Might be different, at least for the oil pressure switch/sender, but that's about it that I can think of.
Then again, 1990 will be VSS, so there will be some added wiring for that, although I suspect the ECM wiring etc stayed the same short of the addition where necessary of additional wires. Which means the cluster wiring is wrong as well.
90 and 91 instroduced alot of changes (idiotically, as Dremu says, "GM is silly" let's change the whole wiring setup for a vehicle we're about to discontinue!).
ABS, elec. Speedo, seat belt sensors, Cruise, stereo, glove box light, among other things.
Could you make it work, sure, is it the same, not by a long shot.