I'm no welding expert, but is it worth considering Brazing? obviously much lower temperatures are involved, -I've done plenty of repairs brazing, though obviously it's different to a complete reconstruction. Didn't they used to solder tanks back in the day?
BSA stopped soldering tanks when they went from flat tanks to saddle tanks around 1920.
Most soldered tanks had a bottom that was like a cup that the top of the tank sat in then the solder was run around the edges
The thick pin liniing hides the joint between the top & bottom,
Notrun used to scollop the upturned sides to get more surface area for the solder
Both joint faces were tinned then they were pressed together & oven baked to get the solder to join
If you look closely at the bottom os an unmolested BSA tank you will see the bottom is rolled over like a tin can lid
If they were tern plate then again oven baking will make a seal
Latter tanks with exposed ends were a rolling resistance weld
These tanks are the ones that rust out along the welds
I am fairly sire that M20 tanks were braized edge on
This would have been a war time idea to save on steel