I generally TIG weld the long-side sections on Buses. I can tell you from experience that the repair you are doing is one of the most difficult to do correctly.
I always remove the floor as it usually needs replacement and it's the only way to access the back of the weld. Also, it's a good idea to planish as you go. That big, flat panel doesn't take heat well. I tack it up. Do a light planish then weld and dress as I go.
Here is the latest one I've done on my personal project. Not a split, but a similar process:
https://www.thesamba.com/vw/forum/vi...=asc&start=100
Cool Mango, semaphore Bus, by the way...