I have a euro V8 trans computer for use with the E/S switch that in effect does exactly what you're intending to do-- it shifts at exactly 7 mph without a spirited push of the pedal form a stand still. I dunno if it's easier to start from scratch or check out the innards of an older trans 'computer'. Any euro V8 W126 should have it.

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1987 560SEL (210,000 miles)