The problem is:

I need more space for backseat cos i want my rear passangers to travel in comfort, with a bit more space for their feet.

So the backseat needs to move back.

This means that the petrol tank needs to be modified,

how do i do it?

what's the safest way,...
I would rather modify the existing one cos a racing tank would take up more space, and won't have a good enough capacity.

If there are any tanks that can be mounted under the car, in place of the sparewheel i would like to know.

Thanks in advance!