Coil sprung series fuel tank options

I have a coil sprung lightweight and the fuel tanks in the back ,it looks to be made from an old copressor and it takes up alot of room

Cant see a standard tank fitting under the drivers seat as the bracket for the rear trailing arms in the way

What are my options ,i have a large space between the chassis legs above the rear axle and im going to make an exhaust that exits somwhere between the axles so that wont be in the way ,but im unsure how id fill it

any suggestions ?

The SIII coiler I used to own had a modified SIII tank fitted in the standard position under the drivers seat. It had been cut and welded to allow the trailing arm to pass underneath.
Only problem was it eventually started leaking on the weld and I then found it could not be removed from underneath without removing the suspension for repair.

Il look into how much id need to chop out of a series underseat tank as that may be my best option