Good Morning Sonny,
Along with the .380 barrel and the complete slide, you will need:
* .380 Recoil System (The springs are different strength - possibly get those at Williams Gunsights or Wolff Springs)
* .380 Ejector (They were wider than the 9mm versions by +/- .007" or so. I believe the R9 ejector was .098" and the R380 was around .105")
I may have an ejector in one of my boxes, so I'll give a look and get back to you.
The springs? . . . . . I really doubt it, but again, I will give a look through what I have.
In a pinch, try using a worn-out 9mm outer recoil spring on your R9 recoil assembly. I did try that back in the day and it did function the slide properly.