Valve body removal is fairly easy first you should go here and download the A727-A804 torqueflite service manual, it gives in detail all about repairing these transmissions.
http://www.mymopar.com/index.php?pid=22If the detent ball is missing you will see that without taking the valve body apart.
If the spring section is in good condition you can order just the ball and possibly the spring to get it back in good working order.
This link is to Jegs for the TCI 220000 shift kit if you click on the instruction s you can download a copy that shows in general how to disassemble the transmission while in the car.
http://www.jegs.com/i/TCI/890/220000/10002/-1Take your time and don't force any parts and you should be ok with this repair.
Good Luck
