I am not familiar with the 3600 series, so use as much or as little of this suggestion as you can stomach. What I did was cut the torque tube just before the pumpkin. Then took the drive shaft out and slid it INSIDE the "cut" drive shaft that connected to my "open driveshaft" rear end. This allowed me to telescope the two different shafts until I got the "right" length. After this was accomplished, I cut the torque tube (at the transmission side) where it just starts to "bell out". This get you the part of the torque tube that supports the closed drive shaft at the transmission and the busshing. You will also need the metal housing the "holds" this part and is bolted to the transmission and the crossmember and test fit it again.Since the "closed" drive shaft was smaller than the "open" drive shaft, I just used 8" pieces of pipe and spacer metal to get a tight slip fit (top and bottom) and then welded it all up. Since, I am not as good a welder as I should be, I then drilled and tapped for bolts to go through the driveshafts & spacer material. I wouldn't even THINK of going 20 MPH or pull stumps. Good Luck!!
