You could also swap a VX/CX tranny in to your car to lower the highway RPMs to around 2,000 at 65mph, this would keep the 4th valve from opening as well as lowering your over all engine speed.
Careful with this, if the engine can't keep up with the load 65mph will put on the engine at 2000 rpm you'll never get that engine down into overdrive and it'll constantly be hunting on the freeway if it even gets into overdrive.
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