Driveshaft help

I am planning on swapping the SB 904 tranny out of my dart and replace it with a SB 727.

The question I have is, How long does the drive shaft need to be?

I have a couple laying around that I can take to the shop to have modified if necessary but I want to have the least amount of down time as possible, and since I need to take the 727 to the shop to be rebuilt for old times sake, I figgered if I could get the drive shaft done around the same time I would not have to let the car sit or do lots of extra work, but I do not know exactly how long I need the shaft to be.

Can anyone with a similar set up let me know how long their drive shaft is?

68 dart, SB 727, 8.25 rear end.

thanks