The reason the stock arrangement of rear sway bars don't work with the larger tank is that the slots for the end links to mount to the 'frame' are behind the tank, so the larger tank, which is wider, is in the way.
I've seen several variations on using the sway bars arranged to point forwards, or to clamp to the leaf springs.
Those 1-1/8" sway bars would be a great thing compared to the stock 1".
If you have front sway bars, make sure they haven't rusted away a lot of their diameter where they pass through the rubber bushings. That would make them quite sloppy.
Heavier leaf springs are probably a good thing, though they'll ride a bit harsher. You can almost bet your originals are the worst for the wear after decades of service.
Please note that new, heavier leaf springs may raise the tail of your van substantially - my B250 maxi van wouldn't fit in my garage after I replaced the rear leaf springs with the heaviest ones which would fit!
I got uprated leaf springs with bushings already installed for my B250 from ESPO Springs 'n Things. They're quite knowledgeable should you have any questions. https://www.springsnthings.com/
You could also investigate Airlift Loadlifter air springs, or Timbren air springs. Timbrens don't require an air compressor.