If you buy from Strange, make sure you get them to fit the axles into the spool before shipment--Thats all.
I have seen both gaskets and sealer used when installing the center section and have different theorys.
Sealer if you dont plan to take it out often and are meticulou when you do to make sure all the sealer is removed. Also dont be a pig about it and use half a cartrage of sealer on it.
Gaskets are good and some even come with a sealing stripe to make them even better, a little gaska-sinch and you are good to go.
Running a gasket moves the center section forward and that in-turn makes the tires toe-out ever so slightly, but since the tires toe-in during excelleration it evens things up.
Before installing the center section-- looking at the housing from the front - The 2 studs to the right of the top bolt should be replaced with bolts( requires an insert to be installed) or at minimum trimmed down to the top of the nut. They have a nasty habit of digging into your calf/drivers suit and if you wear boots, get caught climbing out