This is what I did. I left the outer section of the frame in place. and put the new spring box on slightly to the outside of the frame. I also sprayed and cleaned as much of the old frame as possible and used spray through primer.
On the back side of the new spring box where it contacts the old existing frame I drilled 5 holes so I could plug weld it as well for strength.
I also have an article from a shop that did this same job that I had saved o my computer. If you want a copy PM me and I`ll email it to you.