It shouldn't be any problem getting a stock drive shaft but if you want a HD one Mummert sells them. There has been some talk in the past about the hole in the drive gear on the aftermarket oilpump being deeper than the factory unit and the drive shaft going in too far and not contacting the dizzy. Make sure this is not the case in your application.
God Bless.
Tim http://yblockguy.com/
350ci Y-Block FED "Elwood", 301ci Y-Block Unibody LSR "Jake", 312ci Y-Block '58 F-100, 338ci Y-Block powered Model A Tudor
tim@yblockguy.com Visalia, California Just west of the Sequoias