Here's the fuel tank in the car. It sits back flush to the vertical place formed by some frame members. It's a bit hard to see from this picture, but there are a couple of things that protrude through that plane, and therefore through the firewall skin I am putting there - the filler neck and the vent fitting.
Tubes from both of these will go up and outboard (left in this picture) to the fuel filler on the fender.
Here, I have three panels mocked up to form the lower and middle sections. There's also going to be a vertical upper section lying tight against the upper aft firewall skin (this gets important later), and a small lower center section to cover most of the gap (but leaving a space for shift cables and wiring to pass through).