Bill, If your clamshell doesn't have a visible leak I would suggest checking it once a year by removing the grease fitting on the bottom of the clamshell and inspecting. I do admit I never checked it for 10 or 15 years prior to installing a new clamshell seal. Universal joint was fine. However, I did add another homemade grease fitting to the top of the clamshell that is accessible from inside the car to make checking and filling easy as it is just above the floor mat.It has a red plastic cap in the photo. Keep the zerk fitting finger tight so you can unscrew it and peek inside. Refill to top of of fitting and you're good to go. Also, keep the stock fitting on bottom of clamshell for filling, if preferred..
Yes the "turtle" can be removed from under the car but it's a bit tedious. There is a thin paper gasket. Removing the transmission cover makes it a lot easier. Of course, if your building a concourse restoration, you wouldn't do this.