my frame was a bit rusty towards the rear and the rust is mostly at the bottom of the rails, that is also where the big frame rivets are put to hold the inner rail to the outer rail. I figure if the bottom is rusty the side is probably still tuff so welding the seams is a good idea for strength
I just boxed my frame a month ago.
First I cut out all the old stuff then
welded new steel on.
I did this from the rear engine support to the very back.
Friend of mine did this before,that is what got me going on it.
Seems pretty strong.