Betcha a 555 IC will provide the square wave signal. Somebody with a bit more skill can likely give you the number for a ramp generator IC that will tell the 555 when to change frequencies (or maybe another 555 feeding the one driving the tach input).

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FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered