The recommended spark plug gap for a 1987 Volvo (all models) is .028"
gap is .028
spark plug gap is .028
They should be under the top cover of the engine - no spark plug wires, only some kind of springs that make the contacts.
There is no benefit
You can tune up your Nissan by replacing the spark plugs and spark plug wires. You should also replace the air filter and the oil.