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Unless the 351C in your De Tomaso is equipped with a US Ford manufactured Q code engine, the Q code camshaft is an upgrade.
The Q code cam is worth 30 horsepower to a De Tomaso equipped with an Australian 351C (T code). Worth 40 horsepower to a 1971 Pantera equipped with a US Ford manufactured M code engine.
A 351C assembled using the Q code camshaft (aka the Cobra Jet camshaft) with the compression ratio set at 10:1 with quench combustion chamber cylinder heads to complement the camshaft, metering fuel and air with an optimally tuned (non-emission) Holley 650 cfm carburetor mounted on a standard height dual plane intake manifold would produce 320 horsepower at 5300 rpm if equipped with cylinder heads from an Australian Y code or P code 302C (i.e. 2V quench combustion chamber heads). If the cylinder heads from an M code engine (i.e. 4V quench combustion chamber heads) were used instead that “351 Cobra Jet” would produce 350 horsepower at 5800 rpm.
If a roller cam upgrade is preferred that's a whole other topic. I like roller cam upgrades ... but before we go "sticking a cam in it" I would advise upping the compression ratio first, that's the best way to improve pep and throttle response.
-G