I feel your pain with the complexity of welding the rear tank.
The one I made for my 520exc ended up being a welding marathon.
I combined the air box, battery box, subframe and fuel tank together as on structure all made of 3mm 5083 plate with a bit extra round the mounts to spread the load.
Surprisingly it transformed the handing on the bike, made it much more stable through corners.
I thought it was just because it wasn't flexing like the stock subframe but I'm starting to think its not just because the seat isn't moving but I reckon it's stiffening the main frame as well.
I could be wrong and usually I am.
I thought it would be so simple at the start.
Then it got more and more complicated. I wish I had never tried to incorporate the guard like I did.
This is the only shot of the air box cover that I'm particularly proud of. It took 10hrs to make and works and fits perfectly.
Its been finished and in use for several years now, its survived several failed hill climbs and various other riding errors.