That will depend on a couple of things but it sounds like you will have to have a new driveshaft made. Is the new tranny a manual or auto? Is the rear axle stock (torque tube) or has it been swapped?
There is information on the Empire Driveline site that identifies how to measure for a new drive shaft.
I suggest you call around to a couple of shops in your area and see what they say they need in the way of dimensions to get you a shaft. They are in the yellow pages.