No--don't change anything related to wiring.

Take the 86 block out of the car by removing everything attached to it. That means the intake manifold, exhaust manifold, torque converter, accessories, etc. Pull them all aside and yank the block.

Then, put the 90 block into the 86. You might have to change the flex plate/flywheel, since the 90 has the sensor for the crankshaft pickup on it. But it still should work in the other one, since your 86 won't have a crank sensor.

Attach everything and your car will never know the difference. It'll run happily on the 86 fuel injection system. You should redo whatever marginal wiring there is, while the engine is out of the car (naturally this is a simple thing to do when there's nothing in the engine compartment).
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chris herbst, five volvos.