Rusty rear frame... repair or replace

[JUSTIN72RAT]

I've read several threads on swapping from the lwb to swb. However, I have a different scenario. I have a long box and plan on keeping it long. i don't want to mess with the paint on the box. I do have a rusty frame from about 2/3rds of the way back. The really bad area is above the coil spring area. I believe it is so week that it is buckled in that area making the box sag in the back. I found a frame locally that was cut down to a swb. Why can't i take that and weld it about 12" back from the cab and then weld on the tails of my frame onto this swb chopped frame?