In a Formula Ford the fuel cell ( bladder filled with foam in a aluminum can ) is directly behind the seat. This is the best place for fuel in a crash as if the car compresses enough to crush the tank the ensuing fire isn't an issue.

A fuel tank filled with foam ( to reduce slosh ) is just fine for auto x.

I've road raced a FF in 3 layer + fire proof underwear and it was fine while driving, hot when sitting or after the race, too much of a hassle for auto x.

Long pants are a must for racing this type of car. For a jacket I'd just use a clean welding jacket. Cotton is used because in a fire it will burn to ash where synthetics will melt and stick to skin.

I've used fire proof underwear for track days though the tight fit might gather attention from other racers . However, I did meet my now former sweetie there, she later admitted to wondering what was under there. ( I'm a slim 6 ft 180 Lb )