Hello,
In 1988 Volvo changed the type of wiring connectors used on their cars so the 1986 sender will look different. Also, the ground for the sensor in the 1988 may go throught the connector, the 1986 has a body ground loop that bolts to the chassis near the filler neck pipe.

If the 1988 only has three wires you should be able to splice the 1988 connector to the 1986 sender.

Don't modify the car wiring, change the wiring on only the sender. that way if you ever need to put in the exact part it will plug right in.