Rust Inside Frame

I chopped the back end off my frame a few years ago and have only gotten back to it now. I'm planning on welding it back up but noticed some surface rust inside the tubes where it was cut. It appears it hasn't compromised the structural integrity yet, however I'm unsure how far up the rust has travelled. If I weld the back end on will the rust continue to eat away at the frame? How should I treat this?

Side question: What material should I use for the frame modification?

 

Don't know about material for future mods, but surface rust in the upper members shouldn't be an issue. The real problem is the lower front rails, water gets in there and rots them. You don't know about it until holes appear.

 

there are drain holes in the frame if your concerned about the rust pour some evaporust inside the frame it will drain out and will stop the rust.

mod tubing mandrel over die or something like that is what mod stands for
tcbros.com sells tubes and things

or look on ebay for oldmanmotors he sells hoops got one for a bike xs 650 was what it is for but it is same size

 

I will definitely be ordering from oldmanmotors. Awesome, thanks for the advice.