I agree on getting one without the liner, but.........
How hot can you get it? RED HOT from top to bottom should do it. You can do that with propane weed burner or skip the fire inside the barrel and build a hot ground fire and put the barrel on top of that. A fire inside does a good job on the top half of the drum, but not the bottom. Putting it on the fire does the whole deal. Leave the lid on to trap the heat inside. Once the barrel glows red or close to it, the liner will be gone. Once you heat up the lid, the rubber gasket will fall off.
After it's painted and you season the inside with a test burn, and even more so after one or two smokes, the inside of the barrel will be coated black with creosote, which if my drum is any indication, will only thicken over time.
Also, in my experience, the food grade drums are of a heavier gauge metal, which is good. Again, not the first choice, but all is not lost if that is what you have.
Compared to the work of knocking out a closed head drum, I'd take the food grade and it's lid and deal with the liner.