I have a 365 in 9mm and wondered if I a 380 barrel
would fit in the slide?
Might need different mags and spring maybe?

No. You need a complete upper. Mags would work I believe.

The slide on the 9mm is heavier than that on the 380; as such you would likely have many jams. The 380 slide has three holes to lighten it that are visble if you turn the slide over; in addition the recoil spring for the 380 is lighter.

I actually ran a whole mag of 380 rounds using 9mm magazine with no problems.

The mag for the 380 is identical to the 9 except for a spacer built into the inside back to make up for the smaller 380 cartridge

I love the 365/380 and have adopted it as my daily carry since I do have arthritis in my hands and 380 is pleasant to rack and shoot