.06mg/kg is a pretty small lethal dose. That's probably entirely due to the radioactivity. Tetraethyl lead has a lethal dose of something like 1.2mg/kg http://www.inchem.org/documents/pims/chemical/organlea.htm which is 20 times higher. But that's organic lead, which is rapidly absorbed; lead oxide is something like 600 mg/kg, ten thousand times higher than the dose you calculated for plutonium. But, in that case we're comparing the ingested dose to the dose absorbed; you'd have to ingest about 150mg/kg of PuO₂ to absorb 0.06mg/kg of it.

Also, though, the fatal dose of another heavy metal like lead might be lower than the LD₅₀ reported, because LD₅₀ tests normally don't give the experimental animals several years to die.

So, in conclusion, it seems like accurately comparing plutonium poisoning to poisoning by other heavy metals requires more knowledge than I have. Good thing the NRC's on the job.