If a 240 has a speedometer cable, then the cruise control installed in it gets its speed signal from a small coil that fits into a covered recess in the back of the speedometer. That coil comes out of the donor speedomoeter after you remove the two small screws that hold it in, and you will have no trouble installing it into the receiver car's speedomoter, assuming that that car, too, has a speedomoter cable. Your retrieved harness should have two wires with terminals that connect to the coil on the back of the speedometer. I think those wires are several inches long.

The differential on these cars contains no "speed wheel," so switching to a new differential cover with a sensor in it will prove fruitless.

Typing "240 Cruise Control" into the search window above will yield a few dozen relevant threads that will provide an excellent education on this complex subject.
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jds