Changing from 7 1/4" to 8 3/4" rear end questions

Dana67Dart said
the drive shaft from a stock 7 1/4 is probably a 2 piece shaft with a rubber dampener between the two halves. you would be better off having one made

No, not on a 67-69. It would be solid.

When I did this swap on my 67 Barracuda, I found the correct driveshaft at a wrecking yard (very unlikely today). I had to change u-bolts and plates. I did not have to change u-joints — this was a 741 case with SBP axles — your mileage may vary.