I'm not a chemist so I can't say for sure but if you burn the lined drum and then have it sandblasted it will probably be ok. The hexane from what I've read is used in edible oil extraction so I would think if you do a real hot burn on it, it would probably be ok also. I see people using oil drums after they've been burned out so I would think the same process would make your drums as "safe" to use as theirs. If it makes you feel any better, I don't know what was originally in my first barrel, it smelled like chemicals so I burned it out real hot and let it start to form surface rust and burned it again, took a wire wheel and cleaned it up real good. I haven't grown or lost any body parts to date so I guess it was "safe".
Burn them a couple times. Pallets make a good hot fire. Then give it a good scrubbin with comet and a coarse brush. Add a few parts, and you sir have a new cooker.