I would say no to grounding the fan relay to the positive of the OD relay. You would need a different overdrive relay that has a terminal that does opposite waht the power to OD terminal does... that is, it would kill power when the overdrive gets power.

Or perhaps a second relay on the overdrive circuit that works opposite.. when second relay recieves power from the overdrive being engaged, it would kill power to the fan.

But if you ever stop the car with the overdrive switch on, the fan would still be off even if you were stopped and not in 4th gear. I'd just let the fan do its own thing with the thermal switch that it should already be on. It'll turn off if the engine is cool enough.

Or simply install a toggle switch somehwere so you can operate it manually.

-Matt
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-Matt '70 145s, '65 1800s, '66 122s wagon, others inc. '53 XK120 FHC