I got a final drive off Fleabay, splines are not perfect but OK, but the breather cap is busted.

Here's the fiche --

My understanding from the always-helpful parts folks at Bob's is that 4, 5 and 13 just press in from the top -- no need to open up the case. Has anyone else done this? Looking for red PITA flags here -- I've got 2 weeks to return this thing for a refund.
Also, the unit came with the speedo sensor and cable. I have a working speedo sensor on another final drive whose splines are shot, so if this one is hosed I can swap in the other one. Is there anything in the guts of the final drive that could make the speedo go kaputt? I assume the brains of the speedo are in the sensor and all it does is read some kind of marker on the internal gears to determine how fast they're spinning. Nothing much in the Lieberry on this topic.
My goal is to get it installed and run it for a few hundred miles and see if there is any leakage. Anything else I should be looking for?
As always thanks for any info --
Van