I replaced the one in my bw65 many years ago when it failed a few, months after a rebuild ( still pi#### me off that they would not fix it on warranty. I said forget it then and took it home and did it myself). But I digress. I simply pried out the old and popped in the new. Worked fine. Fun job taking out the transmission on my back with the car on jackstands. Ah to be young again.
Plan A would be to pry out the old seal and replace it with an XJ manual trans the exact same size as your BW66 with the same propshaft and trans mount. You’d have to find a bell and flywheel, unless you wanted the John’s T5 kit bell, adapter and hydraulic throw-out to fit a junkyard T5?
Any/all the above available at shipping cost (for this project only).