50% at a time.

The caps on the seams are not structural. Remove them. Then cut notches
in the seams at intervals of perhaps 6" and cut and grind back the
flange between every second pair of notches, so you've got 6" of seam,
6" unsupported, 6" seam, 6" unsupported...

Weld the missing seam sections up, taking care that the panels are
level. How you adjust this with the rest of the seam intact, I'm not
sure, but if you take the entire seam off you'll never get it right!
Once you've got half the seams cut off and replaced, cut off the
remainder and finish.

The general approach is to run a narrow strip of steel behind the seam.
Grind away a short section of the seam and then weld up the gap using the
steel strip to support the weld. Then repeat the process of grinding away,
etc. If your welding is up to it, you don't need the support strip. But you
will have to be capable of bridging a gap with weld.