Andrew, all of these links aren't what he needs. He wants an aluminum driveshaft, not an original steel one. Perogie isn't a supplier I'd ever recommend either, they are very, very expensive and have been known to misadvertise (or swap) parts on people.
You'll have to check into the aftermarket suppliers. I don't know anyone off hand that has put an aluminum driveshaft in a 68. You may still have to have it customized depending on what trans and rear you have. The yoke or rear may not match up to whatever size U joints the driveshaft has, plus the length will be a factor.