If you can't cut it out and weld it up, you can always try to "epoxy" a patch to it. It won't last forever since that rust likely goes 4 inches in all directions. But the epoxy stuff works if you prep right.
They use it in the trucks episode, but 1/4 of the way through the vid:
https://www.powernationtv.com/episod...two-tone-paint