318 rear main seal replacement

Those are both O rings. The speedo gear can be pulled and the ring replaced without anything else.

The transmission pan has to come down, kickdown and shifter arms off of the shaft and the valve body has to come down, to replace the shifter/ kickdown shaft O ring seal, but that's about a 1 hour job with a beer.

If you've never drained a 904/ 727, get ready. It's going to get all over the place. Just do yourself a favor ~ get a filter kit for the gasket (new filter isn't a bad idea to put in, anyway) and some new fluid, loosen all of the bolts about one full turn, wedge a flathead in the lowest corner (this is where a sloped driveway can actually help you) and twist the screwdriver so it slowly comes out. If you need more, loosen the two adjacent edges of the pan to that corner, before loosening the rest, a little at a time. Get a drain pan with a big catch area that will cover the entire pan area.

When you get the gasket out of the box, put it on something flat, put an old shirt over it and iron it. Then, after you scrape the trans and pan, tac the gasket in a few spots with some RTV and put it on the pan, then tie it up at the corners first with the bolts.

While you're down there and the fluid is out, go over to the passenger side and take the dipstick tube out and replace the O ring on that.

Then, the only old seal you'll have left is the tailshaft to output yoke seal, but if it's not leaking, leave it. If it ever needs replaced, just nose the car down the driveway and you won't have to drop the fluid to do that one.