It would depend entirely on the thickness, and/or taper of the aluminum, the weight of the car, torque of the engine, tire selection, and a myriad of other factors.
If you need an aluminum shaft for a manual race type car, I would buy a new one from a very reputable manufacturer based on their recommendations for your needs.
For a good/cheap upgrade to your average performance B body (Auto or manual), The truck ones have proven to be quite strong. How strong??? I cant' quantify, particularly with a manual "race type" car, but if you don't subject your car to more stress than a heavy truck (that may also be towing something), Their driveshafts work very well as a retro-fit.
I know the aftermarket produces aluminum shafts that will handel massive power, but I don't know if the stock (fitted) truck ones can even come close, but they seem to be fairly trouble free in 600 +- HP big block cars with autos.
As for weight difference, the truck ones are only about 8 pounds lighter than most stock steel B body units (probably near double that for the stock shafts that have the big steel front weights), plus they don't need the huge weights that are attached to some of the transmissions as vibration dampers. --- My guess is that in a best case scenario, the aluminum shaft can save close to 25 pounds over a stock set-up, but probably less.
The aluminum shafts are noticeably smoother running. Nothing earth shaking, but an upgrade none the less, and much easier to find than a stock B-body shaft, as well as cheaper to obtain (purchase of J.Y. shaft, shortening, balancing), than a new custom made steel shaft.
I know this doesn't answer your questions, so I would strongly suggest you contact someone like Dennys, or other reputable manufacturer for specific recommendations if you have a high powered, high traction, manual car. --- Also, please let us know what you find out so we can all benefit.
