The X308 crossmember that I salvaged was always dry and no penetrate used by me , just a fwd looking tip for ease

 

The biggest thing to retrofit the sway bar on X300 is a wishbone tie. Original X300 MNA2855AD ties with bar mounts are hard to find and X308 MNC2855BA is different - no rubber bushings in the middle so mount studs on the differential should be replaced too. And those studs NNC5715AA are surprisingly expensive and also not a lot are offered.
Another big thing is a tie replacement. You must take the springs out and probably have a lot of heat and WD-40 to take the wishbone pivot bolts out (powerful impact wrench is a much here too). Also those studs should you decide to go with the X308 tie are ****ers to remove too. Best way to have it done is to bring the complete rear suspension assembly out.
Third but not last to consider is a sway bar brackets MNA3501AA. Try find used on ebay. Normal price is around $50 a pair (usually comes with rubbers) as some sellers asks 100 each. Or order aftermarkets at SNG. Sometimes wishbone ties are offered for sale with those brackets on. So do a deeper search. Also check on UK ebay too.
Finally before you decide to install a sway bar first thing is to obtain a tie (and hardware for it) and decide if you are ready for its replacement works. The X308 sway bar with upgraded links fits just fine to X300. Also the integrated crossbar from X308 fits directly too.

PS. I was lucky to find the X300 tie from UK seller at affordable price so when i was rebuilding my suspensions this fall i've installed X308 sway and cross bars on my VDP.