The Mustang driveshafts are designed for stock vehicles. In many cases the driveshaft transfers power from the engine/ transmission to the rear axle/wheels and acts as a “fuse” because it can fail before other components when overloaded, the tires hook up, and the clutch doesn’t slip, etc. The driveshaft often becomes the weakest link in the system. You can find the best mustang driveshaft via pstds.com/product/aluminum-mustang-driveshaft.
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Therefore, a primary consideration in choosing a driveshaft is that it is strong and safe enough to not fail. Weight (total and rotating) is a secondary concern. Because there’s little potential for parasitic/frictional loss the only other real considerations are reliability and serviceability.
For a vehicle that is driven primarily on roads with little to moderate modification, the steel driveshaft is hard to beat. The original driveshaft is most likely fine as long as it isn't damaged and the U-connection is in good condition. Almost all first-generation Mustangs used a single steel drive shaft with built-in silencers on the inner surface of the tube.
In most situations, an aluminum driveshaft will perform best per pound per dollar. Used steel driveshafts are not always easy to find in the desired size and design and costs arise quickly if you also have to purchase new shear flanges and new U-joints. For light to medium horsepower, one of the cheaper, pre-fabricated, direct install aluminum driveshafts will cost a bit more than buying and repairing an OEM steel drive shaft.