been there, done that. It was my 84's 380 SEL about 5 years ago. It took me two trials. Initiallly, the car wouldn't go forward. 1st trial got it but it wouldn't shift to 3rd. 2nd trial replaced one of the band(B2?) and it solved the problem. It was impossible to put the planetary unit back together with the new seal unless you have a press. I ended up taking it to a tranny shop and paid $25 for that. Anyway, if you are going to do the valve body later may be you'll be interested in this
shift kit
I'm planning to do it on my 300sd next tranny fluid change