I suggest paying to have the Caddy dyno'ed with the stock drivetrain, and use that as a basis for the minimum hp and torque you should look for.
It's important to bear in mind that before '72 or so, engine hp ratings were gross hp, i.e., without any accessories on the engine (no water pump, oil pump, alternator, distributor, fan, etc), so they were inflated a lot. A more modern engine could be smaller, more powerful, and more economical, even without radical modifications to the car.
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