Fuel cell

[ALMACK]

This is from the Super Stock section in the rulebook regarding gas tanks:

GAS TANKS

Original stock-type fuel tank permitted; must be mounted in stock
location. Tank may be notched or sectioned for suspension
clearance or to fit narrowed rear-frame unit. If filler neck is relocated
into trunk, tank must have pressure cap and be vented to outside of
body. Trunk must be completely isolated from driver compartment
with firewall, minimum .024-inch steel or .032-inch aluminum.
Accessory tank(s) prohibited. Fuel cell permitted, minimum 3-
gallon, maximum 16-gallon capacity; may be mounted in trunk. Cool
cans, fuel blocks permitted. Only one fuel cell or tank permitted.

See General Regulations 1:5.