Polyethylene is approved for fuel tank use BUT only with gasolene or diesel approved fillers and fittings. Not sure what plastic is used in food-grade barrels (we get them for horse training obstacles), it may be polypropylene (which I think disolves in gas!) or some other plastic. I definitely agree with the other respondents that use of food-grade barrels for fuel storage is not a wise idea. I'm sure if there were a safe way to turn a cheap used plastic barrel into a 55 gallon fuel tank rather than spend hundreds for a steel, aluminum, or polyethylene one, someone would be out there selling them.
VistanTN
40 gal polyetheylene tank from Northern Tool is $399.99
cheapest 55 gal steel tank I can find is $189.99
we pay $8 for the barrels, empty, but unrinsed.