one piece driveshaft??

[NoHachi]

Just use stock unless you are forced to go custom driveshaft. All the theoretical babble ends up amounting to nothing much noticeable, besides the vibrations when the pinion angle is out.
Its not light enough to make any difference (and the weight is right where you want it). The rotational inertia is identical, certainly compared to the flywheel. Lots of other places where you can spend the money.

If you do buy one, adjustable 4 links are a must, as is andrews article about setting the angles.

Just another opinion to add to the bunch.