Crossmember over rear coil spring and frame rust

[Atown]

Hello all-Recently got a 1971 c10 longbed that I’ve since converted to a short bed. While it’s been apart I’ve noticed some heavy rusting where the rear crossmember and the bottom frame rail are riveted together. It’s also bad where that reinforcement piece inside the frame rail sits. Ive tried doing some searching on the topic but haven’t found anything definitive on the subject. Does anyone have any suggestions on the fix? Is it better to find a section of frame to weld in or can I replace sections of the fram rail? Thank you in advance.