You’re going to need to brace the cantilevered rear spring mounts, replaced the rotted out inner frame rail with new metal, buy patch pieces to replace the last foot of the frame, the triangles under the frame, rear crossmember, & angled braces.
EBay or @Awl_TEQ are likely your best bet for frame replacement parts. I’d buy from @Awl_TEQ if possible as his parts are excellent quality. Although I’ve not physically laid hands on them they appear to be 2nd to none.
This is my recommendation based on the information you’ve provided… with more details we can give you more guidance.