Panacur is a relatively safe anti-parasiticidal so I agree that higher doses (100mg/kg) are probably pretty safe. I'm not sure I would use that high of a Flagyl dose though (especially if we are talking about Drymarchon). Most of the literature on Flagyl also recommends a dose of 25-50mg/kg. "Understanding Reptile Parasites" by Dr. Klingenberg recommends that dose and cites Dr. Funk in specifically saying: "Do not exceed 40mg/kg in tricolor snakes, indigos or Uracpam rattlers."
I'm not saying 100mg/kg of Flagyl would kill your indigo...it probably wont. But why take the risk when a much lower dose will accomplish the same results? Chris