No tach drive on Series II but they kept the same cover, hence an o-ring and plate arrangement. I use two O-rings there, one flat and one skinny as the groove is too wide for the usual 1/8" o-ring supplied, the skinny one just holds the fat in position laterally.
Looking at eBay it appears they sell a half-moon filler that should let you use the cam cover w/o the opening (I have never done this).
Cometic gaskets work great but I didn't like the fit of the ones made for the cam covers with the gaps at the rear -- got a perfect fit by getting the gaskets that are solid across the rear (from the sedans) and cutting my own gap.
Some cam covers have long notches for a cross-over tube -- only 1969 US spec Series II and European engines did not have those notches so when I needed a new cam cover for mY 69 S2 it was hard to find -- in the end got one from the UK where they are plentiful. Just saying, if yours have no notches know that the replacement you are considering may have them.
In addition, late cam covers use a stick-on label for the 'Jaguar' rather than having it cast in the cover -- but maybe that is preferable for your application.