On a 69 Plymouth GTX 727 trans, I have a trans leak coming from the vertical gear selector shaft where the shift linkage connects to and goes inside the trans. Is the seal accessible by dropping the trans pan or is it something that you need to drop the entire trans to repair?

You can get to the one selector shaft seal without removing the pan. You will have to remove the linkage. Actually there is a special tool for removing the seal but I have done it before without one. You can pry it out with a small screwdriver and tap the new seal in with a socket. Some people have used a c clamp to install it.

If you drop the pan, you will need to drop the valve body also. I usually drop the tranny crossmember and tilt the back of the tranny down with a floor jack to give me additional room. That way you don't have to drain the tranny fluid or have a big leak from the tailshaft (leave the driveshaft installed). Remove the linkage (kickdown and gear selector), clean the area to keep stuff from falling in when you remove the seal, and pry the seal from the case with a small screwdriver. To install the new seal, lube the seal lip with tranny fluid. Get a socket that will fit the metal part of the seal and clear the shaft, and tap the seal into place. Reinstall the gear selector and kickdown linkage. Top off tranny fluid as necessary and take it for a test drive...

Another way to do the install, after pulling pan and dropping valve body, is to get a piece of all thread and a couple nuts and washers bigger then the seal, set the seal into position. Stick all thread up from the bottom, put a washer on the seal and a nut started on the threads, now do the same on bottom side and start tightening the bottom nut. This will pull the seal into place with out dropping crossmember and less risk of damaging the seal. Works great for me.