There's actually 2 seals on the speedo cable housing, one on the inside and an o-ring on the outside.
Looks like the picture below. You can buy a kit that includes both of them.
Speedo is pretty simple, just pops out. Just the retainer with one bolt holding it as pictured above. Not much to the kickdown seals, but like the speedo there is an inner and outer. I did mine with everything dissasembled. Probably need to drop pan and valve body to do in car. but not sure. Outer is a reg oil seal you'll need to tap in, so may be tough to do with trans installed.