My three C3's have drain plugs in their transmissions and rear ends so it's always really easy to replace their oils. Drain plugs are very easy to install and they're one of the things all C3's need. When I replace the 85W140 rear end lube in my '71 tomorrow I'll take a couple of pictures of its drain plug so you can see how I did it.
It's really easy to install drain plugs in the transmission's pan or rear end and they are one of the best modifications for any C3. When I change my rear end lube I'll take a couple of pictures of my drain plug so you can see how easy it is to install. Just requires a 1" Harbor Freight drill bit for the hole in the center strut rod bracket and a 7/16" drill bit for the hole prior to tapping the 1/4" NPT threads for the Allen plug.