Drive shaft length?

Working on a 66 Dart that I changed the rear to an 8 3/4. Will be using a 904 trans. I moved the rear springs in 3" and the replacement boxes have 2 mounting points for the rear springs. Right now the springs are in the original locations but I might move them to the rear mount which will move the rear back another inch adding to the wheel base. Now tonight I put the drive shaft in and push it all the way into the trans then pulled it back so the u-joint was mated into the yoke. It came back about an inch. If I move the rear to the rear mounting holes it would be about 2" from being bottomed out in the trans. What are your thoughts about the travel distance of the drive shaft?
Rod